Go back a few hundred years and teenagers were effectively considered adults. In some places this is still true. My cousin took a trip overseas to somewhere really underdeveloped and met an 8 year old who was running a shop pretty much on his own. He did his own inventory, stocking and he ran the register. My cousin isn't easily impressed but I've never seen him more impressed than when he talked about this adult child.

These days young people -- especially those in first world countries -- don't fully mature until halfway through their 3rd decade. This is partly because because teenagers are coddled but mostly because teenagers are actively discouraged from making adult decisions. When I was child I lost count of the number of times I saw a TV show or read a book or heard a story where teenagers were cast in the role of a villain. I was terrified that turning 13 would magically turn me into a "bad" person and I committed myself to not making any decisions on my own.

Children don't magically turn into adults after turning 18 or 21. Adulthood is only achieved once adult decisions are made and the consequences of those decisions are lived with. If teenagers aren't allowed to make at least some adult choices on their own then it is only natural that they won't be adults when they move out.