The current study examined the age of learning effect on second language (L2) acquisition. The research goals of the study were twofold: to test whether there is an independent age effect controlling for other potentially confounding variables, and to clarify the age effect across L2 grammar and speech production domains. The study included 118 Mandarin-speaking immigrants and 24 native English speakers. Grammar knowledge was assessed by a grammaticality judgment task, and speech production was measured by native English speaking raters' ratings of participants' foreign accents. Results from the study revealed that the age of learning effect was robust for both L2 domains even after controlling for the influences of other variables, such as length of residence and years of education in the United States. However, the age of learning variable had a stronger impact on speech production than on grammar. The current results support the framework of multiple critical/sensitive periods (Long in Int Rev Appl Linguist 43(4):287-317, 2005; Newport et al. in Language, brain and cognitive development: Essays in honor of Jacques Mehler. MIT Press, Cambridge, MA, 2001; Werker and Tees in Dev Psychobiol 46(3):233-251, 2005).