Why are so many JRPG characters reported as much younger than they actually should be? There's got to be a reason.
From what I've heard, high school occupies a similar place in the Japanese psyche that college/university does in the West— the time in your life where you get a bit more freedom and responsibility, gain maturity and perspective, and start to figure out who you are and what you want to do with your life. With that in mind, the young ages of your JRPG heroes begin to make sense: a character being 16 to 18 years old— high-school age— communicates that they are at an important juncture in their life (important for a coming-of-age story), and a character being a bit older than that (say, 19 to 21) suggests that they've gone through that stage of their life already. Their accomplishments and backstories may be incredibly implausible for their age, but it's not about what's "realistic" so much as what makes sense
emotionally.
(Sidenote: while I did criticize Lulu and Celes for their backstories being too "compressed", my underlying point was really about that emotional aspect. Lulu is jaded and world-weary in a way that I don't associate with the age of 22, but can buy for, say, 28.)
But, like Peklo says, we should also be aware that we're operating within our
own cultural bias, too. Western media tends to prefer older protagonists— "older" relative not to Japanese media, but relative to the intended audience. Superheroes tend to be written as being about 30 years old, give or take;
Star Trek captains are generally in their 40s, as are
Doctor Who's leads; and while
Lord of the Rings has screwy ages that don't exactly correspond to real ones, the principal players are certainly all adults. (There's probably a lot to unpack here, as well, about our own proclivities towards having protagonists who are independent and/or feel like authority figures.) So although Japanese heroes skew young to begin with, our own cultural perspective amplifies it.
