No Stat Atrophy

In Role Playing Games, once a stat is increased it will usually never decay on its own accord. If a character uses a bow, say, and uses it a lot for a while his archery skill will increase. If he subsequently never uses it again, the skill level will remain constant no matter how long it was since he used it last. Games that track the characters' age will usually not simulate the weakening effects of aging.

This is one form of Acceptable Breaks From Reality; most players would find it tedious to constantly use various skills to prevent them from decaying. Often it's compensated by the way stats scale up as the game progresses, such that enough archery skill to kill same-level enemies at level five can't scratch them at level 20.

On the other hand, from one game to the next if there isn't a good in-game explanation, it seems the characters really let themselves go.

As this trope is almost always used in video games, only exceptions will be listed.

Dating Sim

 * The H-Game True Love had several stats which would influence random encounters and events, if you didn't keep these up then you'd be slacking off and miss out on some interesting plot.
 * Shira Oka Second Chances decreases points in skills that aren't necessary for the particular activity you're doing each week (i.e. if you choose to study art and literature, you may receive a slight decrease in your math skills until you choose math as your next activity), so it's important to try to switch up your activities every week to keep from falling behind.

MMORPGs

 * In EVE Online your skill points can decrease drastically upon death unless you've prepared a clone, which costs a tidy sum.
 * Having a Strategic Cruiser blow up around you will decrease a rank in a (random) SC skill (the skill setup for the ship type is... complicated). Ejecting before it blows or self-destructing will avoid this.
 * The Sims Online features stat decay. You can train your skills like you could in the offline version, but after your total amount of stats reach a certain amount, your untrained stats will begin to decay.
 * Ultima Online averts this, to the point that while the game was popular a number of people paid for people to play their characters while on vacations or business trips to avoid losing grand master status.
 * In fact, for a while after launch it was worse than that. You had a total skill cap, so if you had 700 skill points worth of skills, any skill raising will mean a random skill was lowered. However, they also had "learning by watching"- if someone used a skill near you, there was a chance you would learn some of that skill. Cue people creating macros to spam lighting campfires at Britain Bank and causing maxed out people around them to lose points in their grand master skills. Learning by watching didn't last long, and eventually UO implemented "skill locks" where you could designate which skills you don't want to have ever raised or lowered.
 * In Final Fantasy XI, you can lose experience points (and levels if you lose enough XP) through death. Crafting also has skill decay: you can level all crafting skills to 60, but you can only level 4 crafting skills to 100 (i.e. you have 160 more skill levels available). If you try to take a crafting skill beyond 60 after using up all your 60+ skill levels, one crafting skill would lose a level for every level you gained.
 * Combat and magic skills also "decay" when switching to lower level jobs: every job level has a cap for a particular skill, so even if you have a level 90 Warrior with maxed sword skill at 325, switching to a level 1 Paladin will limit your sword skill to 5.
 * This is only temporary, however. The game keeps track of your actual skill level, and merely caps the "effective" level until the job you're using get high enough. To use the above example -- Paladins actually have better 1-handed sword skill that Warriors do, so they will regain access to those 325 points in the skill at some point before they reach Paladin level 90. As a bonus, up until that point, it also means that they instantly jump to the next level's cap as soon as they gain a level in the "new" class (here, Paladin); thus, they don't have to worry about having sub-par skill in 1-handed sword for their level for a long, long time.
 * When World of Warcraft implemented weapon skills, neglected skills did not decrease, but level scaling meant that if you maxed out Axes at, say, level 40, and then picked up an axe again at level 60, you'd have 20 levels worth of skill points to catch up on, during which you would be pitifully ineffective. As this proved annoying, working in practice to discourage changing weapon types rather than as a fun mechanic (Oooh, look, a shiny epic dagger. Now I have to go spend 3 hours grinding my skill up!), Blizzard dropped it in patch 4.0.
 * Dynasty Warriors: Online Has all your stats reset at the end of the battle. You need to pick up flasks, the arguable Booze-Based Buff in the game, to return your strength to where it was at the end of the last battle. This is mostly used to balance fights.

Miscellaneous Games

 * This trope is mostly played straight in Echo Bazaar, with the exception of the Admired quality, which will drop over time.

Platform Game

 * A variation: the Chao virtual pets in the Sonic Adventure Series have values and experience levels for each of their stats. Feeding them certain fruits and letting them play with animals (not including yourself) will raise their experience in some stats and lower them in other stats. However, the experience level of a stat cannot drop below what was necessary to previously level up that stat. This creates the interesting situation where it's actually not beneficial to be balanced in what items you give, instead spamming a whole lot of one item and then, once the stat in question levels up, spamming another.

Roguelike

 * In Nethack stats can be exercised and abused, causing them to rise and fall accordingly. For example, starving makes you feel weak, eventually causing your strength to deplete. However, losses due to abuse are considered the same thing as stat drains from monsters, and can therefore be "cured" in certain ways.
 * Memorizing a spell from a spellbook imparts it to memory for a fixed amount of time, after which is must be re-memorized. In the spin-off Slash'EM, however, reading a spellbook commits it to memory for a much shorter amount of time, but every time you cast the spell boosts how long it will stay memorized, so a spell you use all the time will only need to be memorized once, while spells that are rarely used will quickly fade from memory.
 * Starting in DF2010, Dwarf Fortress implements a very rough system of skill decay: a dorf who doesn't use a skill for a while will become "rusty", and then "very rusty". Most effects of this aren't known, but it is known that a dorf who stays "very rusty" for too long will suffer permanent experience loss in that skill.

Role Playing Game

 * In the original version of Final Fantasy II, building up magic would reduce your weapon abilitys and vice versa, but this was removed in the GBA version.
 * In Final Fantasy X, some Blitzball players would actually lose stats on some level-ups. Extremely minor and rare, however.
 * Also in Final Fantasy VIII, where increasing your affinity with a Guardian Force to summon it faster came at the expense of losing affinity with others.
 * In Final Fantasy IV, the old sage Tellah increases his Intelligence and Spirit when he levels up, but his Strength and Stamina decrease. This is meant to simulate the effects of his old age.
 * Final Fantasy V has a status ailment called "Aging" which weakens the character. Fortunately, it's not as permanent as the name of the status ailment implies.
 * Realmz always had character aging, but it didn't use to have any effect. In version 5, each race was given age groups (youth, young, prime, adult, senior) which alter stats when a character enters that age group. In order to avert whining from players, characters past their race's maximum age didn't die, but instead merely got a large penalty on XP gains as a hint that the character should be retired.
 * In Darkstone characters can and will age, and once you hit a certain age your stats begin to decrease. There are potions in the game which can reduce your age to make you younger and stronger, but they are extremely rare and hard to find.
 * In SaGa Frontier 2, Wil Knights loses half of his stats due to old age in his granddaughter Ginny's scenarios, but they can be built back up via prodigious grinding.
 * In Oblivion if you are put in jail, and serve your sentence without escaping, some of your stats will drop.
 * In Lost Kingdoms II, the main character has an affinity level for each of the seven elements of cards used. The maximum of said level was 8 (I think, it's been a while since I played), but no matter how much you tried balacing card usage, only one element could attain (and keep) this level at any given time. Somewhat justified, since several maxed-out affinities would be too gamebreaking.
 * In Disgaea, reincarnating will cause your character to lose some of their weapon and skill proficiencies. Spending more Mana will ameliorate the losses, but you can still potentially forget spells if you reincarnate repeatedly without practicing the spell.

Sports Game

 * In the Mario Tennis and Mario Golf series, not leveling skills would make them gradually decrease.
 * In the Inazuma Eleven series, every character has an individual total stat cap. After hitting that cap, any further stat training will cause another stat to drop by an equal amount as an Anti-Grinding measure.
 * MLB The Show, in the "Road to the Show" mode, if you don't work on something every two months, your ability will begin to decrease.

Tabletop Games

 * Some tabletop RPG systems, such as Dungeons and Dragons: Characters can age, decreasing their physical stats while increasing their mental ones, and there are many spells and monster abilities that can permanently drain statistics or even levels.
 * Order of the Stick lampshaded an odd side-effect of this: since Wisdom is undeniably a mental stat, and therefore increases when you age, and since perception-type skills use your Wisdom stat, your vision and hearing actually get better with age, as opposed to real life.
 * However, skills rarely decay so if you keep training at a faster rate than you're losing stat points you can still be the best swordfighter in the universe even if you can barely stand up.
 * At least until you die of old age.
 * The XP penalty seems to be a nod to this trope, although it's not related to aging. In 3rd edition, if you have levels in two classes and neither is favored for your race, you receive a 10% experience penalty. The assumption is that when you're not adventuring, you're training to keep up your skills, and this becomes more difficult if you're training in multiple disciplines.
 * In GURPS your stats drop as you age and start dropping faster as you reach the limit of your species lifespan. If a stat reaches zero the character dies naturally, though in practice a stat below 5 can quickly be lethal even in domestic life.
 * Technically skills fade if you unused (unless you have an Eidetic Memory) but that's usually a bit of book-keeping that no one finds necessary.
 * In the latest edition of the BattleTech RPG both physical and mental stats start to drop past middle age. Additionally the character will eventually receive the Glass Jaw (increased vulnerability to damage and fatigue) and the Slow Learner (increased exp cost for all skills) traits. Clan characters even take negatives to their Reputation to reflect the ageist sentiments within their society. All of these can be countered by spending experience but eventually characters will see a slip in their performance to the point that they die of old age when their BOD hits zero.

Turn Based Strategy

 * As far as aging goes, Final Fantasy Tactics kept track of the ages of characters by how many calendar years had passed since the game began. If you spend a lot of time traveling and battling, the three-year-old Prince Orinas may be 20 by chapter 2, but he's still an infant as far as the story (and his appearance) is concerned.
 * A similar situation in Final Fantasy Crystal Chronicles. You may have played the game to year 157, but your character's appearance- that of a 12-15 year-old- doesn't change at all.

Web Comics

 * Played with in the MS Paint Adventures story Problem Sleuth. As this fake Adventure Game becomes increasingly more of a fake RPG, there are only three main stats for the three main characters - "imagination", "vim" and "pulchritude". Each character starts out with one stat insanely high ("pulchritude" for Problem Sleuth, "vim" for Ace Dick and "imagination" for Pickle Inspector) and the others virtually empty. Throughout the "game", each time a character gains a level their stats go up - but only the one they're already proficient in. Their other pitiful stats might go up by a point or two, stay even, or drop - at one point Pickle Inspector actually had negative vim.

Wide Open Sandbox

 * GTA: San Andreas. Physical exertion will consume the character's energy, first in the form of his reserves of body fat. If that is used up, his muscles will start to decay, making him weaker.
 * This is also an example of why it's rarely used. Many gamers complained about having their wanton violence, setting things on fire and parachuting off of high buildings interrupted by mandatory visits to the nearest restaurant. It was removed in Grand Theft Auto IV despite it being the most realistic of the series otherwise, possibly due to complaints.
 * In ~Sid Meier's Pirates~!, all your stats (but swordfighting and dancing in particular) decay as your pirate ages. Choosing Medicine as your preferred skill at the start of the game reduces the effect somewhat.

Literature

 * An unusual variant occurs in David Brin's novel The Practice Effect. In it, it's the tool itself that must be used often, lest it decay back into its original, probably very primitive form. Why bother spending a lot of time and effort crafting something nice when simply using the bare minimum will improve it dramatically? On arrival, the protagonist finds a extremely high-end ax and takes it with him in a rucksack, but after a few days of not using it, is surprised to discover it has turned into a barely serviceable chunk of wood and metal. In this world, the upper classes hire whole teams of people to 'practice' their belongings and keep them at a state of perfection.