Three groups of participants were given a grammaticality judgement test based on five structures of English grammar in both an oral and written form. The first group consisted of native speakers of Chinese, the second, native speakers of Spanish, and the third, native English speakers. The two learner groups were divided into those who had begun learning English at a younger (less than 15 years) or older (more than 15 years) age. Performance was measured for both accuracy of judgement and time taken to respond. The results showed that performance patterns were different for the two learner groups, that the linguistic structure tested in the item affected participants' ability to respond correctly, and that task modality produced reliable response differences for the two learner groups. Although there were proficiency differences in the grammaticality judgement task between the younger and older Spanish learners, there were no such differences for the Chinese group. Furthermore, age of learning influenced achieved proficiency through all ages tested rather than defining a point of critical period. The results are interpreted as failing to provide sufficient evidence to accept the hypothesis that there is a critical period for second language acquisition.