In most cases I never found it odd that RPG heroes could do the things that they do. Generally you don't play some completely run of the mill kid in RPGs, you play an exceptionally talented, exceedingly determined individual who probably has some sort of power that is strange even by their own world's standards =P Maria from Xenogears, for example, isn't a normal child at all. I don't want to spoil anything, but if you've played the game you'll know why.

Talent is talent, and I never found it strange that somebody with that sort of talent could save the world in RPGs just as I don't find it strange that, in real life, there's probably plenty of child prodigies out there that are well beyond the aptitude of many adults at certain things. I guess there are some exceptions, such as Justin from Grandia who is...basically just a normal kid, but generally RPG heroes aren't exactly run of the mill children.

What I find stranger than kids being able to save the world in RPGs, though, is them being able to take the mental strain of such a burden. I mean basically if the kids fail, then the world is completely doomed. At that sort of age I couldn't even fathom that sort of burden, it'd simply be too big. I'd be terrified. There's no way that as a kid, even if I had RPG hero-esque powers, I'd be able to handle that sort of pressure. Heck I doubt I could handle it now ^_~ Yet most RPG heroes swallow it without so much as a second thought. I always found them having that much courage to be stranger than them having any amount of strength, personally ^_^