The ways in which age of acquisition (AoA) may affect (morpho)syntax in second language acquisition (SLA) are discussed. We suggest that event-related brain potentials (ERPs) provide an appropriate online measure to test some such effects. ERP findings of the past decade are reviewed with a focus on recent and ongoing research. It is concluded that, in contrast to previous suggestions, there is little evidence for a strict critical period in the domain of late acquired second language (L2) morphosyntax. As illustrated by data from our lab and others, proficiency rather than AoA seems to predict brain activity patterns in L2 processing, including native-like activity at very high levels of proficiency. Further, a strict distinction between linguistic structures that late L2 learners can vs. cannot learn to process in a native-like manner (Clahsen and Felser, 2006a; 2006b) may not be warranted. Instead, morphosyntactic real-time processing in general seems to undergo dramatic, but systematic, changes with increasing proficiency levels. We describe the general dynamics of these changes (and the corresponding ERP components) and discuss how ERP research can advance our current understanding of SLA in general. (Contains 5 footnotes, 2 figures, and 2 tables.)