Question: why is it so easy to learn a language as a child?

Nayeli Gonzalez-Gomez answered on 14 Sep 2017:

The primary assumption is that language development is just like any other forms of growth and development. As such it would have what’s called a critical period. This is a particular period of maturation in which, with the appropriate external stimulation, the ability will pretty suddenly develop and mature. Before that and later than that, it’s either harder or impossible.

In experiments with animals, it’s been shown that if they’re deprived of stimulation, for example, a kitten doesn’t get patterned visual stimuli in the first several weeks of life, its capacity to develop vision would be permanently damaged. But if pattern stimulation does appear at that time, then the kitten will develop normal vision.

It could be that there is a critical period for language acquisition, roughly from birth to around puberty and that if you don’t learn language at that time, it’s going to be very difficult to learn it. If you have learned a language by that time, you will be able to learn more languages later, but probably in a different way, and maybe by connecting it to the languages that you do know.

This is a very good video explaining how babies are able to learn their language: