If a person has not had a chance to acquire his target language by the time he's an adult, he's unlikely to be able to reach native speaker level in that language.
外国語を学ぶ一番良い方法は、ネイティヴスピーカーから学ぶことだと、大抵の人が言う。
Most people say that the best way to learn a foreign language is to learn from a native speaker.
本学を卒業した学生なら誰でも最低2年はネイティブスピーカーに英語を習っている。
Every student who has graduated from our university has studied English with a native speaker for at least two years.