Back in Old Testament times where people, on average, lived to be several hundred years old (Noah was over 600 when he started building the ark), a boy was considered a man and a girl was considered a woman when they turned 12.
Now, with an average life span of less than a century, we don't consider people adults until they're at least 21...more like 25.
Something just seems wrong with that.
There's a difference between being a kid at heart and still being a kid. It seems most of the people around the late teens/early 20s mark that I know are the latter. And I don't think that's how God meant it to be:
"When I was a child, I spoke as a child, I understood as a child, I thought as a child. But when I became a man, I put away childish things."-1 Corinthians 13:11
By the way, my friends Alex and Brett have written a whole book on this subject. If you haven't read Do Hard Things yet, you should. It's mostly geared toward teenagers, but people of all ages will find it helpful. I have a copy that I'm willing to lend. :) Boundless.org also has some great articles on this geared toward the college-aged crowd, including The Peter Pan Syndrome.