Adults are very different beasts from the children they once were. Becoming an adult is far more than growing older. A adult has to take responsibility for all their actions and in doing so many of them feel they have to put away childish ways. They close themselves off from the open-minded playfullness and exploration they once knew as a child, forgetting that life was meant to be enjoyed, not suffered through.
None of my friends are adults. They are all responsible children. And, I wouldn't have it any other way.