This is an interesting subject. To me it seems that the expectation of being a grown up at a certain age is just part of the propaganda. I don't think that people really ever were mature at the age of twenty, and most of them used to have children at that age. But the way it's been with your extended family living in the same house, the parents didn't have to be all that mature and wise, since there were the grandparents (at their forties or something) who had already seen a thing or two. But now that the extended family is cut off, the children will not have mature people around them, they just have their child-parents.