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(1) Adults proceed through early stages of syntactic and morphological development faster than children (where time and exposure are held constant). (2) Older children acquire faster than younger children (again, in early stages of morphological and syntactic development where time and exposure are held constant). (3) Acquirers who begin natural exposure to second languages during childhood generally achieve higher second language proficiency than those beginning as adults.
Krashen et al. (Sat,) studied this question.