.....The youthfulness of the protagonists, even in first person perspective games, is an issue in cut scenes, where you often do watch/view them. It also is an influence on the character in terms of their background and in watching other characters interact with them. A young protagonist is more of a blank slate (if he or she suffers from memory loss -- which happens occasionally to adventure and casual game protagonists -- he or she is even more of a blank slate

).
I suppose that younger people are more adventurous than older people in general? So that, for a game that involves a quest, it seems more realistic to send a younger person halfway across the world, up mountains, into caverns, etc. and expect them to survive?
Maybe youth makes a protagonist seem more vulnerable, which makes it easier to empathize with them as characters. Maybe older protagonists would be too experienced to believe the
letter in the parcel that sends them to the haunted house?

I personally find protagonists more interesting if they have some life experiences that have shaped them. That may not be the norm, however.