It seems to me that its largely due to the fact that our society doesn't let you do anything until you are 18, sometimes 19. For example, when my parents were teenagers, you could get your full drivers license at 16, now in British Columbia, you get your learners license at 16, your 'N' at 17 and you don't get your full drivers license until you are 19.

Also in British Columbia, you can't rent an apartment without someone signing for it until you are 19. So its hard for someone to grow up until they are in their late 20s because they are used to society protecting them until such a late age and then when it is time for them to move out etc. they don't know what to do with themselves because they haven't been able to learn what to do when they are on their own.

So if our society wasn't so over-protective, people would be growing up faster, but instead we are treating them like children until they are almost 20.