Dual Age Modes

There are a lot of characters who can change their shape. Some have fewer options than others, and a lot are limited to only a handful of forms. One common set includes an aged pair: the same general appearance, in a childlike and an adolescent or adult form. Often, one will be the natural form for the character, with the other being invoked by some external trigger. But most of the time, the child form will be used only to make the character look more innocuous or relatable.

See also Older Alter Ego for a child who gets powers in adult form, and Sleep Mode Size for a powerful adult Mode Locked into a child form. If the character just ages from childhood to adulthood by normal means, it's a Plot-Relevant Age-Up or Time Skip. If a child is forcibly transformed into an adult, it's Overnight Age-Up; the inverse is Fountain of Youth.

Anime & Manga

 * Signal in the manga/anime Twin Signal has two forms: An adult Bishonen form, and a childlike Super-Deformed mode with an insane love for chocolate.
 * Miss Hinako in Ranma One Half has the default form of a girl of about eleven years, and turns into a curvaceous twenty-something by absorbing Life Energy, Battle Auras and Ki Attacks. Although it's never addressed in the source material, it seems likely that her adult form is her true age, and her child form is the result of slowed aging due to Happosai giving her Energy Absorption powers via altering her metabolism with Pressure Points.
 * Happosai's old friend Rakkyosai is cursed to change into a small child after falling into the Spring of Drowned Boy. The thing is, he's exactly the same (toddler-like) size in both forms, so he has no advantages or disadvantages from one body to the other, but his real body is more... wrinkly. A lot more.
 * In Yu Yu Hakusho, Koenma sometimes appears as a toddler and sometimes as a young man.
 * Juugo (Naruto) turns into a child when he gives Sasuke some of his flesh to heal him; he turns back into an adult after absorbing one of the samurais.
 * In Tenchi Muyo!, Ryo-Ohki is a shapeshifter with four forms: a spaceship, a "cabbit" (cat rabbit creature), and child and teenage humanoid forms- the last of which is hard for Ryo-Ohki to control or maintain.
 * Also Washuu, who can take any form she pleases, but spends most of her time as a little girl.
 * Nel in Bleach returns to her adult form by absorbing energy from attacks.
 * Vivio and Einhart of Magical Girl Lyrical Nanoha Vi Vid both have their normal, elementary school forms, and their teenage forms they use in combat.
 * from Higurashi no Naku Koro ni. It won't become very obvious before the second season.
 * Age misrepresentation pills in Mahou Sensei Negima, and whatever magic Eva uses. Mainly used for disguise, the only combat effect is the change in reach.
 * Space Pirate Mito is an alien that looks like a third grader, despite being over ten thousand years old, but has a mail suit that she uses for combat that looks like an adult human. She has similar suits for just masquerading as a human adult, and a good deal of the conflict in the first half of the series comes from trying to get her half-human son to accept her actual, childlike, form, when for most of his life he's only seen her in mail-suit form.
 * In Ah! My Goddess, Belldandy mentions that the three goddesses are each a representation of time. Belldandy represents the Present, so when she uses too much of her power, she just needs to sleep. However, when her sisters use too much power, Urd, representing the Past, reverts into a child form, while Skuld, representing the Future, grows up into a teenage form.
 * In Hell Teacher Nube, both Nube and Tamamo (extremely experienced spiritualists) can use astral projection to generate child-like bodies for themselves while their real ones lie in a sleep-like trance. Nobody ever seems to recognize them despite looking nearly identical to their adult forms (although Yukime thought child-Nube was exceedingly cute.)
 * In Hayate the Combat Butler, Isumi's great-grandmother can take child form by drinking other people's blood; when the effect wears off, she reverts to her true form. Like Rakkyosai above, there's not much difference between her two forms, apart from her true form being really, really wrinkly.
 * Jewelry Bonney of One Piece does this. She could change her age to any other amount too. And does change the ages of others as well, as a combat technique. She and her crew wear undersized clothes, possibly so they become right-size when they're changed into kids as a disguise.
 * The slightly obscure manga(?) 1520 has an interesting version of the trope. The two main characters, a 15 year old prince and a maid from opposing kingdoms, eat a magical cake by accident that transforms them into ten year old children. Whenever the maid laughs or the prince sheds a tear, that individual will return to their proper age, at the cost of de-aging the other down to about five.
 * In Millennium Snow, Yamimaru (the vampire's bat companion) can take on human form as either a strong young man or an adorable little boy.

Comic Books

 * Shazam.
 * Thunderbunny. (Lesser-known 80s comic in which a boy transforms into a grownup anthro rabbit superhero).
 * Skaar, one of the Hulk's sons, can change between a small gray child-like form, and a bigger Hulk-like form.
 * Similar to Captain Marvel, The Ultraverse's Prime. He also had a Distaff Counterpart called "Elven."
 * One of the Heroes tie-in comics had The Russian, a Special who posed as an entire crime family with his ability to physically age and de-age himself at will.

Film

 * Dave Bowman in 2010.

Literature

 * Aphrael in The Elenium and the Tamuli. Being a goddess, she can look however she wants, but generally takes child form because it makes her easier to adore.
 * Foster from Keith Laumer's A Trace of Memory goes from an old man to a youth in the space of an evening... but loses his memory in the process. It turns out this happens to him  on a regular basis. The plot is motivated by his friend's quest to restore Foster's memory, and The Reveal has to be seen to be believed: it's one of the cheekiest twist endings in all of literature - cleverly foreshadowed, but still quite shocking in its chutzpah.
 * Hecate from The Secrets of the Immortal Nicholas Flamel goes from being a maiden in the morning, a mother during the day, and a crone by the evening. It should be noted that she does not have control over the process, however.

Live Action TV

 * Big John, Little John -side effect of a sip from the Fountain of Youth.
 * Power Rangers Turbo: Justin after transforming into a Ranger. Sometimes averted, when he doesn't wear his helmet. Justified because of the footage of Carranger.
 * In Power Rangers SPD, there's Mora, who starts the series as a child, then partway through is transformed into an adult named Morgana (which is actually her natural form), before being turned back into Mora near the end.
 * Kibaranger in Gosei Sentai Dairanger does things the same way as Justin.

Video Games

 * Link in The Legend of Zelda Ocarina of Time, by way of Time Travel. Also in Twilight Princess, who spends most of the game in Sleep Mode Size.
 * from the game Tales of Symphonia.
 * The titular Space Ace in the laserdisc video game is turned into a child as a result of the Big Bad Borf's "Infanto Ray". During certain parts of the game, the player has the chance to "energize" Ace and turn him back to his adult form for a short time; adult Ace sequences tended to be more about fighting villains while child Ace spent most of his time trying to avoid or escape enemy attack.
 * of Ar Tonelico became a child when she lost the Chronicle Key.
 * Pollin, the magical girl parody from Tech Romancer has her standard "little girl" mode and occasional older version.

Web Comics

 * 21 year old Evan Onymous of The Wotch can transform into a 4-year old girl called Lily ("Lil E"). (Also a fully-grown female version, Missy ("Miss E").)
 * Pandora from El Goonish Shive appears in her child form when she reveals herself to mortals, and assumes her adult form when dealing with her son and during her later appearances in the ephemeral universe parallel to the physical plane of existence called the spiritual plane. These are not her only forms however; as a shapeshifter she can appear as anything between a monstrous "cloud" and a clone of any other character.
 * Mr. Raven can alternate between a sixtyish form and a twentysomething one.

Western Animation

 * Clockwork from Danny Phantom constantly shifts between different ages due to his time-related powers.