Scott Young writes about Seven Expert Opinions I Agree With That Most People Don't. I like most of them, but #6, Children don't learn languages faster than adults, deserves a closer look.

Some people imagine babies have some magic language ability that lets them pick up their first language easily, even as we adults struggle through verb conjugations to pick up our second. But babies are embedded in a family of first-language speakers with no other options for communication. If an English-speaking adult was placed in a monolingual Spanish family, in a part of Spain with no English speakers, after a few years they might find they'd learned Spanish "easily" too. So Scott says:

https://astralcodexten.substack.com/p/critical-periods-for-language-much