They speak it very well.

I have met very few people who've learned a second language as an adult who speak without a trace of an accent and exhibit other minor, grammatical problems.

A child learns a language by immersion and not by study. The mechanisms are different. They learn the actual rules that govern the language as it it used by its speakers. Later, they are exposed to other usage rules to insure that they write a language closer to the standard. A child also learns a great deal of the grammar of a language in a span of five years or so.

Ceci n'est pas un seing.