Fwiw what you're referring to with respect to age is called the critical (or sensitive) period hypothesis. In fact there are measurable differences in kid's abilities to distinguish sounds where the difference is not phonemic (needed to correctly understand their native language) from 10 months. R and L in Japanese (lateral vs central alveolar approximant), dark/velarized vs clear L in English (lady and bold have different Ls), word-final voiceless/voiced consonants in Dutch/German ("haff you seen my kitt?") etc. As far as we can tell, most kids have no problem learning a new language if they get some input before about 6-7, and then after that it depends a lot on the individual