Critical period is a term in language acquisition theory that refers to the time in a person’s life after which full acquisition of the first/second language can be problematic. For hearing people this time is considered to be around puberty. For the deaf, the best time for acquiring a sign language ends after age of five.

The question of critical period is important for sociolinguistic course as it deals with acquisition of either first or second language. Improper acquisition of a language (including ASL) is characteristic of a certain group of language users.