yeah, the japanese almost always use teenage protagonists. Scandinavia and the World did a comic about that:http://satwcomic.com/child-s-play
In the west, we prefer our hero's to be nicely matured, like a good scotch or wine. Seriously, you see a teenager in a western action movie or game, he is either A: going to die quickly, or B: cause a lot of trouble to the main character in order to keep alive.
I can see how younger protagonist might make sense when they are silent, as their lack of dialogue can be explained away by their innocence and inexperience. But now that main characters are more and more frequently talking, you need them to actually have something of substance to say, and the fact is, substance takes some time to develop in characters. I think Final Fantasy 13 proved that. All the characters that had anything useful to say/useful in combat were finally over the age of 20. I think you'll start seeing more protagonists being aged in their 20's in the future, possibly in Suikoden 6.