Sentence processing in American Sign Language (ASL) was investigated as a function of age of first language acquisition with a timed grammatical judgement task. Participants were 30 adults who were born deaf and first exposed to a fully perceptible language between the ages of birth and 13 years. Stimuli were grammatical and ungrammatical examples of six ASL syntactic structures: simple, negative, agreement verb, wh-question, relative clause and classifier sentences. As delay in exposure to a first language increased, grammatical judgement accuracy decreased, independent of ASL syntactic structure. The signers were less accurate and responded more slowly to ungrammatical as compared with grammatical stimuli, especially the early and delayed first-language learners in comparison to the native learners. The results held across grammaticised facial expressions, signed markers and verb type. These results, in conjunction with previous findings, indicate that the onset of first language acquisition affects the ultimate outcome of syntactic knowledge for all subsequent language acquisition. (Contains 4 tables and 4 figures.)