An enduring question in the study of second-language acquisition concerns the relative contributions of age of acquisition (AOA) and ultimate linguistic proficiency to neural organization for second-language processing. Several ERP and neuroimaging studies of second-language learners have found that neural organization for syntactic processing is sensitive to delays in second-language acquisition. However, such delays in second-language acquisition are typically associated with lower language proficiency, rendering it difficult to assess whether differences in AOA or proficiency lead to these effects. Here we examined the effects of delayed second-language acquisition while controlling for proficiency differences by examining participants who differ in AOA but who were matched for proficiency in the same language. We compared the ERP response to auditory English phrase structure violations in a group of late learners of English matched for grammatical proficiency with a group of English native speakers. In the native speaker group, violations elicited a bilateral and prolonged anterior negativity, with onset at 100 msec, followed by a posterior positivity (P600). In contrast, in the nonnative speaker group, violations did not elicit the early anterior negativity, but did elicit a P600 which was more widespread spatially and temporally than that of the native speaker group. These results suggest that neural organization for syntactic processing is sensitive to delays in language acquisition independently of proficiency level. More specifically, they suggest that both early and later syntactic processes are sensitive to maturational constraints. These results also suggest that late learners who reach a high level of second-language proficiency rely on different neural mechanisms than native speakers of that language.