It all comes down to the Official Rules of Japanese-Made RPGs. Evil comes back every 1000 years (set your calendars!), girls with amnesia always try to marry the prince but fail, fifteen is the perfect age to have the athleticism, mental toughness, and expert swordsmanship training to save the world, and anyone over thirty is a hopeless fogey a half-step from dropping dead of old age.
(Though come to think of it, it tends to apply to anime, too. Lina Inverse? The girls of Bubblegum Crisis?)
Y'know, that's got to be based on some kind of market demographic studies. There's no way that game companies can keep cranking out games starring barely pubescent heroes without some data indicating that that's who's buying the games. I know that when I actually was that age, I liked when heroes of various media were my own age so that I could better identify with them.