First language acquisition differs from second language acquisition in prelingually deaf signers: Evidence from sensitivity to grammaticality judgement in British Sign Language

► Deaf native signers, early and late learners judged BSL sentence grammaticality. ► Early learners performed worse the later they were exposed to BSL. ► Late learners' performance was not affected by age of learning BSL. ► Unique effect of age of learning BSL found in early learners. ► Prelingually deaf late learners may benefit from first language competence in English.