It seems that only with Cro-Magnon Man (homo-sapiens) did ‘childhood’ take on any special meaning. Other animals in the wild mature within two or three years, and are then able to fend for themselves. With the advent of modern man came a longer period of childhood development. And that remains so even today with all of us, homo-sapien-sapiens. :)
The big brain of the human causes a big head which requires ‘early birthing’, before the head gets to large to pass down the birthing canal. Because the human brain is so immature at birth, it requires a long childhood.