I remember reading that learning a new language before puberty is much more efficient than after.
This is how young children pick up their native language from that spoken around them.
It’s not just efficiency. Childhood language acquisition occurs in a different part of the brain from adult language acquisition, and the child language part of the brain is basically designed to create language.
That’s why children can learn languages (yes, plural) easily and essentially perfectly, while adults struggle and are seldom able to achieve true native proficiency.
It’s also why children who grow up isolated (like those stories of horrific child abuse where the kid was kept in a closet for their whole childhood) never really develop proper language skills.
And it’s also why groups of children that grow up without exposure to a “proper” adult language (as happened in colonial times*) will automatically
as a group fill in all the gaps in grammar and vocabulary!
And it’s how some sign languages developed: deaf children in institutions couldn’t speak, and started signing. And the magical child language brain created everything needed for complete language. (It is believed that American Sign Language significantly evolved from Old French Sign Language, which was one of those spontaneously-invented sign languages.)
*Like during the colonial era, where there are numerous examples of groups of immigrants, with no common language, who are forced to work together. The adults muddle their way through with very broken language (often, all speaking broken versions of a language that is foreign to all of them), using incomplete sentences, irregular grammar, etc. which is called a “pidgin”. But the children of those adults hear the pidgin and automatically fill in the gaps, and the resulting language, which is grammatically complete, consistent and stable, is known as a “creole”. After that, the creole continues for generations as a stable language.
I just think it’s amazing that it literally takes ONE generation to completely make a proper, complete language out of pidgin. Our childhood brains are literally built to embrace language.