A lot of them already do though? I mean sure, lore is mostly dropped after the first month and vtubers generally just stick the thinnest veneer of character over talking about their own life experiences, but if you look at just the model and the character (and disregard any "actually a 900 year old dragon" nonsense) then the vast majority of vtubers are "playing" a 20s-or-earlier woman, even when the talent is actually significantly older than that. Marine (or Clara, or almost any other popular "hag") may joke and banter about being old, but ultimately the surface-level character is under 20 and her model looks it, and I don't see her ever wanting to change that, and I think the same will apply for most talents who really are early-20s as they grow older.I would find it weird if a vtuber I watched suddenly started behaving like a younger, less mature woman, complete with a younger-looking character model to boot. It would also be antithetical to the character arc we want to see of talents we watch - instead of them maturing, growing, learning new things and gaining new skills, they're age-regressing instead. That would feel really weird to me, and I don't want to think that a talent is going backwards in their personal growth. I don't think many talents would suddenly want to start roleplaying someone much younger.