A study was conducted to identify the scope and timing of maturational constraints in three linguistic domains within the same individuals, as well as the potential mediating roles of amount of second language (L2) exposure and language aptitude at different ages in different domains. Participants were 65 Chinese learners of Spanish and 12 native speaker controls. Results for three learner groups defined by age of onset (AO)--3-6, 7-15, and 16-29 years--confirmed previous findings of windows of opportunity closing first for L2 phonology, then for lexis and collocation and, finally, in the mid-teens, for morphosyntax. All three age functions exhibited the discontinuities in the rate of decline with increasing AO associated with sensitive periods. Significant correlations were found between language aptitude, measured using the LLAMA test (Meara, 2005), and pronunciation scores, and between language aptitude and lexis and collocation scores, in the AO 16-29 group. (Contains 6 figures, 7 tables and 11 notes.)