We investigated the extent to which second-language (L2) learning is influenced by the similarity of grammatical features in one's first language (L1). We used event-related potentials to identify neural signatures of a novel grammatical rule – grammatical gender – in L1 English speakers. Of interest was whether individual differences in L2 proficiency and age of acquisition (AoA) influenced these effects. L2 and native speakers of French read French sentences that were grammatically correct, or contained either a grammatical gender or word order violation. Proficiency and AoA predicted Left Anterior Negativity amplitude, with structure violations driving the proficiency effect and gender violations driving the AoA effect. Proficiency, group, and AoA predicted P600 amplitude for gender violations but not structure violations. Different effects of grammatical gender and structure violations indicate that L2 speakers engage novel grammatical processes differently from L1 speakers and that this varies appreciably based on both AoA and proficiency.