The leading sentence is: > The critical period hypothesis[1] is a theory within the field of linguistics and second language acquisition that claims a person c

The leading sentence is: > The critical period hypothesis[1] is a theory within the field of linguistics and second language acquisition that claims a person can only achieve native-like fluency[2] in a language before a certain age. Granted, I do admit I missed the subtlety that it applies to learning a first language as well.