Yes, as we age, the brain starts to think. In fact, as we age, our daily conflicts and challenges increase, and this causes our minds to constantly wander to other things. Children sleep easily because they are not busy, but we adults face thousands of challenges. I wish we could clear our minds like a child!
I think it might be the opposite for a child - because they are always busy even though they have no obligations. They are constantly thinking, learning, pushing themselves.