Fleeting Demographic

""I remember going to arenas in the 80s, where the audience 60% female and they were all trying to kill us for beating up Ricky Morton. Those people spent the same amount of money as the guys did but now the audience is 80 men and whatever women those men can drag to the matches with em because it's their night out. So we have completely shut off... where else do find a business featuring a bunch of good looking guys wearing very little clothes and no women want to see the show? Riddle me that Lucy!""

- Jim Cornette

A show intended for a certain audience with a clearly defined demographic. Essentially, this means the audience may begin to watch the show at a certain age, but at some point will abandon it later, presumably in the future. This applies to many shows for children. The 'original' target audience simply outgrows the show.

Such shows usually do not provide any deliberate treats for a Periphery Demographic. Because anyone watching will not do so for a long time, the showrunners can be tempted into repeating many premises they know the current audience will consider "new".

Anime and Manga

 * Takara & Sunrise's Brave Series of Super Robot shows is an interesting case, as they originally set out to produce something that would attract a new generation of kids each year, but wound up developing a dedicated fanbase anyway. Though the sixth series, The Brave of Gold Goldran, the most "kiddie" of the bunch had the highest TV ratings, it was unpopular with longtime fans and was apparently a merchandising disappointment as well. After that, the final two shows went for Multiple Demographic Appeal, first with Brave Command Dagwon, where the robots were piloted by a team of teen heartthrobs in an attempt to cash in on the earlier Samurai Troopers series' Periphery Demographic success with teenage girls (a scheme Sunrise would again pull, this time with much greater success with Mobile Suit Gundam Wing). Then came Gao Gai Gar, a giant love letter to the last two decades of giant robot anime which found much more success with the otaku crowd than it ever did with schoolchildren.
 * Dagwon actually had a slightly different problem - it was made to cash in on Evangelion, and the overall feel of the stories was frequently closer to Ultraman than anything else. If anything, the target audience had to grow up a bit to appreciate it.
 * Much of the longevity of the Pokémon anime can be attributed to the fact that the target demographic cycles in about five years so the fact that the same general plot is used every generation only serves to annoy some older fans (mostly outside Japan).

Comic Books

 * Superman comics in the 1950s and 1960s used to repeat the same plots every three to five years, since that was the average length of time a child would read comics.
 * The introductions of Mon-El and Star Boy in the Legion of Super-Heroes are well-known for outright copying earlier stories, to the point where the story that became the Mon-El story was most likely reprinted in the Superman in the 50's book specifically because this made it famous.
 * Archie Comics works on the same premise of recycled plots as Superman, etc; they have kept the Riverdale gang in high school for over 65 years now...
 * Strangely, it seems that the majority of its readers nowadays are people who have been reading it ever since they were kids.
 * Treasure Chest, by definition, as it was distributed exclusively through Catholic parochial schools.

Live-Action TV

 * Barney and Friends, Teletubbies and most other "preschool"-oriented shows. While earlier examples like Sesame Street had enough Parental Bonus to benefit from the Nostalgia Filter, the success of Barney led to a trend in the '90s of making these shows as plotless and repetitive as possible, out of the belief that that made them more suitable for the target audience. Now that those original toddler "fans" are teens and young adults, they want as little to do with these shows as their parents and older siblings did.
 * Lately, some of these preschool shows have started to have small but still existant Periphery Demographics, simply due those who watched the shows as kids are now nostalgic for them. It certainly helps that some merchandise featuring these characters (Barney in particular) are aimed at the people who grew up on the shows.
 * One of the reasons MTV has maintained its popularity over time is that it elected not to follow its original audience, the teenagers of the eighties and early nineties, into adulthood, instead opting to always focus its marketing on the current generation of teenagers. (Whether its quality has also been maintained is a matter best left for debate elsewhere.)
 * The Simpsons lampshaded this by having a quick reference to Logan's Run that involved microchip-implated VJs that alerted the execs to send in a new show host whenever the original person became a Christmas Cake.
 * The Disney Channel has a policy about cancelling a show after 65 episodes. The only show to date that has avoided that cancellation is Kim Possible, due to huge fan outcry (and, allegedly, the contract with the German network that ran the show). The idea is that 65 is the number you need for syndication, and kids will probably watch repeats anyway, so why make more? Also, since kids' tastes change so fast, they expect that no child would stick with a show for more than three seasons regardless.
 * It's worth noting that the probable reason for Kim Possible being such a popular show is its Multiple Demographic Appeal.
 * The endless tween sitcoms on Disney Channel (the oldest of them only had a span of slightly more than four years) are going this way as well. Of course, that's about how long people can convincingly play "tweenage" without going into Dawson Casting.
 * Power Rangers: Although the show does have its older fans, the general demographic is young boys who will grow out of the show after a few years, which is probably why the show started doing the new storyline every year thing instead of the continuing storyline it used to do. Since this is exactly what happens in the Japanese couterpart, Super Sentai, no one seems to mind.
 * In the UK, twee/teen dramas Grange Hill and Byker Grove both had this. Both shows ran for long enough that their original target demographic became utterly periphery, but never completely left the show behind. Unfortunately, the BBC execs decided to shift the target demographic down towards even younger children, resulting in a complete loss of interest from all demographics, and the eventual cancellations of both shows.

Magazines

 * Mad Magazine has lasted fifty years by being read by the newest crop of preteens who initially fall in love with the publication and then a few years later finally grow out of it to complain that "it isn't as funny as it used to be".
 * Saturday Night Live has a very similar relationship with its audience.

Music

 * Hello, Miley Cyrus! Or Hilary Duff! Or . . . well, pick any tween musician. They're going to be around for a couple years until their current fans get older, at which point the next one comes along.
 * Most of these girls turn out to be their own fleeting demographic.
 * As hinted at above, Disney has made a cottage industry of manufacturing tween pop stars to work like dogs for four years before discarding them for the next kid. This is frequently derided as "the Disney Music Machine" in the press and has been going on for at least 60 years.

Newspaper Comics

 * Non-TV example used as a plot point in Pinky and The Brain: Bil Keane's Family Circus cartoon seems to fall squarely in this section.

Theatre

 * In-show example: Bye Bye Birdie has a scene where a barely-teenage girl is sad because by the time her idol Conrad Birdie comes back from the army, she'll be too old for him.

Video Games

 * Happens with most Edutainment Games, especially the ones specifically made for kids of a particular grade level.