How come most anime characters are teenagers?

[Rorschach]

I generally go with a combination of the "puberty sells" and "the main demographic is geeky young males" theories in explaining the prevalence of adolescent guys as the main characters in these stories. Comic books here in America have almost nothing else in them these days, though the companies are trying to branch out to women and children again. Japan, by comparison, does have a much broader fan base, though most of its animes continue to appeal to the emotions of youth and puberty (in girls as well as boys).

The great thing about anime is that although it does have a mainstream, its broad market allows for a lot more niche appeals. Hence, we get brilliant works for more mature viewers such as Cowboy Bebop (which featured an episode devoted entirely to middle-aged men!) and weird experimental stuff like Serial Experiments Lain. There's something for everyone.

I imagine as Anime and Manga get more popular here, our own comics and animation companies will benefit as well, as soon as they learn how to follow the trend. So far, American animations have begun to capture the artistic style of anime, but not so much of its writing and plotting. When they learn how to play to a more mature audience without insulting its intelligence, they'll have a hit.