I don't think the age matters as much as how the writers develop the characters. Sure, they might generally be too young to "saaaave the world" but the same could be said for anyone from any age group. Personality defines a character more than age. Tender age is just a factor that theoretically makes a person more vulnerable, but it's not like adults are totally immune.
I find that in most late SNES and PS1 to present era jrpgs, that the protagonists aren't necessarily just there to "saaave the world." (Legend of Zelda is an exception) Often, the adventure just starts as one motivated by a personal motive, vendetta or thrill-seeking. Is that so hard to believe for a kid? Then, circumstances usually propel them to go on their "world-saving spree." Mostly, it's still motivated by personal reasons and saving the world just happens to be a necessary consequence of accomplishing a more personal end. Sure, it's sometimes absurd how someone that young could possibly persevere through such a journey, but most jrpg stories equip them with an equalizer anyway. It could be a unique ability, character trait, support from friends or any number of other factors.
I find jrpgs more enjoyable than gritty D&D inspired stories. They're aesthetically idealistic and it works that way.
Also, these characters do break down sometimes and I find that it's in how they overcome the odds that makes their characters more appealing - more fitting to be the hero that saves the world (Cloud, Luke)
If you really want your characters to be older, there's always Saga Frontier II where some of the characters reach their 70s in some scenarios (Old enough for ya?)
@dott.Piergiorgio: Umm... but that's assuming that everyone in the industry and the target audience is in the same age group as you, though.
I think P3 and P4 are bad examples though. They both have silent protagonists and all the other party members are high school students who act too much like adults. In most jrpgs, there's usually a hardened veteran in their 30s to 40s or two in the party.
As a conclusion, any mention of tvtropes.org leaves a bad taste in my mouth, but I like my jrpgs with a little bit of these http://project-apollo.net/text/rpg.html