I’ve written lots and even made a video pointing out a very common mistake made by academics and laypeople alike on the topic of second-language acquisition.
This is the conclusion that language learning is inherently more difficult and less successful with age, based on the observation that the older one begins to learn a new language, the worse the results tend to be.
What this assumption ignores is the vast differences between what adults and young children learning new languages typically do and experience.
Adults generally consciously study and practice new languages before they’ve even had much exposure to them, while children pick up languages implicitly, listening and understanding a lot before speaking much.
However, research and the experiences of many learners show that with a lot of comprehensible input—language presented in a way that’s understandable—adults too can pick up language without instruction as children do.