I think the fact that older people have more perspective and can give good advice is true. But (in my 50s) I probably have more perspective than in my 30s. I was certainly a fully grown adult by then.
This brain chemistry thing is, I think, vastly overblown. Apparently, one’s white matter peaks around age 40 (so, is still developing until then). Are people going to claim, on that basis, that adulthood begins at 40?!
Young adults are less risk averse than older adults, but that has pluses as well as minuses. Are we really going to say that most mathematical and scientific advances were made by children (most are under 25).
This is all very academic until you see the way some parents impose an extended childhood on their children by babying them. If you write all their professional e mails, personal statements etc etc for them, and do not allow them to grow up by making mistakes and learning from them, of course, at 21, they will present as children.