It should be noted that the critical period hypothesis relates specifically to acquiring language, and not acquiring a language.

Meaning that if their first language is not acquired by a certain age then the person will not be able to ever fully master language entirely.

This is unrelated to learning a second language and does not mean that people over a certain age will never be able to achieve fluency in a second language.

The majority of unsuccessful attempts at learning another language are caused by ineffective teaching and learning methods along with other important factors such as motivation and necessity. Adults are able to learn languages just as well as children do given the right circumstances.