Here's How Old You Can Be And Still Master a Foreign Language, According to Science
Don't give up yet!
The new analysis suggests that we remain very skilled at learning the grammar of a new language up until the age of 17 or 18...
However...:
"If you want to have native-like knowledge of English grammar you should start by about 10 years old,"...
https://www.sciencealert.com/window-for-mastering-new-language-wider-than-we-thought