Hmm ... I have a one-shot where the characters are not-particularly-young (or old)-seeming people of ambiguous age, and wrote a 5-pager with a clearly old deuteragonist
My main series has an under-30 MC though XD
I mostly came here to say the main issue I have with younger protagonists is it kind of feels unrealistic for them to have mastered very difficult skills (e.g. in combat, magic, etc) and I can certainly suspend my disbelief to enjoy the story, but it still feels very obviously 'fictiony', like it reminds me that I'm reading a story (ah yes, the author made the main cast young bc the story is aimed at a young audience/they need to be kawaii for marketability purposes)
And often, it also makes amazing superhuman abilities/feats feel 'cheap', for lack of a better term; like I don't get that sense of wonder/'scale'; I guess the best way to describe it is, we kind of take computers for granted not, but if someone from 3000 years ago were told that 3000 years into the future, there would exist these devices that do [these things computers do], they would be in awe. Similarly, I feel it gives powers a sense of weight if the only people who have it are people who have lived for decades