Humans are rather a special case, for a baby makes great demands on the parent both before and after birth. A human baby looks precocial. But few of the bits are working, and a baby is completely helpless on its own. The key fact is that the cerebral cortex is not yet working properly. Nerve growth is still going on. During the first 18 months the cortex is getting larger and getting into working order. Then, after about 18 months to two years, the infant starts to learn language and from then on there's no stopping them.