Because 16-20s make sense.
Youngsters going on adventures and living out a dream of adventuring through a naive look of the world.
When you hit 30s, the reality of life becomes much harsher.
And with an older age, a lot of JRPG refocus their story and character development based on it.
For what it's worth, this is one of the biggest deterrents for me in many role playing (irrespective of region) games, or games in general actually. The notion that only young teen (or borderline teen) folk can embark on adventures, riddled with their naivety, has naturally withered in appeal as I've gotten older and learned the same lessons they relearn with the subtlety of a sledgehammer to the face.
I'm far more interested in stories that revolve around a character not adventuring "because they're young and have much to learn", but because they find themselves in a complex situation that forces them to make tough decisions
and have the developed the intellect to approach those decisions with the depth possessed by a mature adult.
It's about context, and the context for a young person adventure is so frequently lazy in its excuse. When age becomes a factor, and the difficulty of their journey more pronounced, the story as a whole for me becomes much more intriguing.
This is doubly so for characters who have history that can integrate into the story too. When your protagonist is so young you're quite often dealing with a blank slate. A boring entity shaping their identity throughout the story, much like a coming of age adventure. Older characters can not long be challenged with whatever the adventure and struggle is going forward, but be burdened by the past that lead them to develop into the person they are at the beginning.
Much of this is why I like Papa Nier so much. He's an tired old dude fighting to survive and save his daughter, not some wanky young shit on an adventure to get his first kiss and defeat mean mr teacher.
