Speaking a late-acquired second language (L2) involves increased cognitive demands, as has been shown mainly in young and middle-aged adults. To investigate grammatical inflection in older L2 speakers, we acquired behavioral and functional magnetic resonance imaging data, while L1 and L2 speakers performed a grammaticality judgment task. L2 speakers showed higher error rates than native speakers, specifically when incorrect forms had to be rejected. Poorer performance in L2 speakers was accompanied by increased activity in the medial superior frontal gyrus (SFG), indicating the additional recruitment of executive control mechanisms. In addition, post-hoc within-group comparisons of behavioral and neural correlates provide evidence for dual-mechanism models in older adults, suggesting that language processing involves both procedural and declarative memory systems. Moreover, we demonstrated that speaking an L2 requires more executive control and relies to a lesser extent on the procedural memory system than speaking one's own native language.