I think our 20's is a weird period too. At that point we still romanticize "being young" and we are perhaps reluctant to fully settle into adulthood. I know in my early 20's I did sort of feel like I was "not quite an adult yet". I also had not mastered things like managing my money more frugally and eating more healthily. It was probably just a case of arrested development on my part!
I agree "teens" are a weird group to define because depending on the culture they're not always coddled youngsters. I dunno, remembering how I was as a teen it's difficult to see teens as outright children. Naive young adults? Most certainly! The same league as 6-12 year olds? Not remotely!
But I do think teens need time to grow up and decide what they want out of life. Granted, there are adults confused about what they want out of life too but at least adults have more skills and wisdom at handling the uncertainties.
Then again, if a 40 year old guy and a 18 year old girl (Or vice versa) can somehow "make it work" I'm not going to judge them. Teens are at least capable of knowing right from wrong and that their decisions have consequences.