An extended childhood has definite advantages.

Many scientists claim that species with lengthy childhoods are more intelligent, and some even claim that being childlike was a necessary step in our development. Chip Walter, in an article in Slate from 2013, writes:

Our extended childhood essentially enables our brains to better match our experience and environment. It is the foundation of the thing we call our personalities, the attributes that make you you and me me. Without it, you would be far more similar to everyone else, far less quirky and creative and less, well … you.
But it's not only being childlike for a longer period of time. Being born with big heads could have presented a very serious problem - like not getting born at all. Walter explains:
More than a million years ago, our direct ancestors found themselves in a real evolutionary pickle. One the one hand, their brains were growing larger than those of their rain forest cousins, and on the other, they had taken to walking upright because they spent most of their time in Africa’s expanding savannas. Both features would seem to have substantially increased the likelihood of their survival, and they did, except for one problem: Standing upright favors the evolution of narrow hips and therefore narrows the birth canal. And that made bringing larger-headed infants to full term before birth increasingly difficult.

If we were born as physically mature as, say, an infant gorilla, our mothers would be forced to carry us for 20 months! But if they did carry us that long, our larger heads wouldn’t make it through the birth canal. We would be, literally, unbearable. The solution: Our forerunners, as their brains expanded, began to arrive in the world sooner, essentially as fetuses, far less developed than other newborn primates, and considerably more helpless.
None of this, of course, explains or justifies extending childhood well into what's usually viewed as maturity. That, I guess, is best explained simply by a lack of wanting to grow up.

Go to: The belated Bar Mitzvah Boidem.