Language acquisition. Two main sides Behaviorist (Skinner) and Chomsky (biological perspective.
Chomsky:
All children pass through the same stages of language acquisition suggesting that it is innate and an inbuilt response.
All children begin their speech with variegated babbling which is the same no matter which language and by the age of 6 months they start to produce sound in their native tongue.
Young children learn grammar in the same way. (Hallidays 7 function and Jean burks wug test)
There is no trigger language acquisition happens naturally. E.g. even victor (the wild boy of Averyron) could produce sounds, he just mimicked wolves and other wildlife as that is what he had grown up with.