In the UC anime/movie line six of the series had teenage protagonists while four had adults with Char's Counterattack (which I only add because by this point Amuro is an adult even though his popularity comes from the first series) and the other three are the 08th MS Team, 0083 Stardust Memory, and MS IGLOO.
Then we get into the alternate universes with Gundam Wing (teenage), Gundam Wing Endless Waltz (teenage), Gundam Seed (teenage), Gundam Seed Destiny (teenage), Gundam Seed Stargazer (not teenage), Superior Defender Gundam Force (teenage), Fighter G Gundam (not teenage). Combining all of that with the manga, books, games et al and its easy to see why an older protagonist is something of a novelty, particularly when every other non-teenage character was in his twenties.
Besides that I'm a bit conflicted. On the one hand I consider UC Gundam to be the best of the 'verses, on the other I wish they would decide to focus on another 'verse to be expanded instead.