One the one hand, the human brain and the human skeleton are both said to have reached maturity at 25 or a little under that. By "maturity" I mean "no longer in a stage of development as is associated with growing up". As a mother who has three children who have passed 25, I feel that's about right. When each child was eighteen I noticed that I didn't feel quite ready to "just back off" and think, "It's his/her life, and this isn't my worry at this point." (That's not saying I don't worry about them even now, but I'm no longer kind of "mentally monitoring" their choices and wishing they'd do what I wish they'd do because.) When they were 18, though, I was always conflicted over not being entirely ready to "let them go" but knowing they were 18 - not 6. The conflict gradually decreased with each year that passed, and by the time they were 23 I noticed I'd almost naturally and gradually gotten "much better" about "backing off" (even if my thinking was something I tried to keep to myself). By the time they were 24/25 I felt as if they were "complete adults". Why I bring this up is that I find it interesting that brains finish developing at that age, and mothers (at least some of us) can seem so naturally to finish letting go when they're around that age too. Having experienced this myself, I feel as if Nature seems to build into mothers its own kind of growth and development that coincides nicely with the growth and development of children.
Having said that (and on the other hand), there are children and teens who go through some experiences that turn them (in one or more ways) from "kid" to "adult" long ahead of schedule (sometimes mentally but also sometimes physically. For example, stress has been linked to early physical (reproductive) maturity in humans (but even in female ferule cats). The kid who is more "mentally" or physically mature is obviously still not completely adult, but the point is that sometimes kids can reach aspect of being adult far longer than Nature planned (but also as the result of how Nature, itself, works).
It's worth mentioning, though, that adults continue to grow as well, although not physically. Getting older with each passing year, having children, going through any number of life experiences, etc. etc., are all things that contribute to more and more mental/emotional maturity.
Worth noting may be that it is around age 30 that metabolism begins to slow down (which would suggest that 30 is when people no longer require the same kind of metabolism needed for earlier growth). In spite of what I said about 25, I have to say that I do see 30 as "truly established as a grown-up".