The current study set out to examine the age-related effects on ultimate attainment of second language (L2) phonology and grammar. The goals of the study are threefold: (1) to unravel the complexity of ultimate L2 attainment by surveying multiple contributing factors, (2) to explore the relative strength of the Age of Arrival (AOA) variable and other predictors in relation to L2 outcomes, and (3) to test the validity of two specific versions of the Critical Period Hypothesis, i.e., the Doomed-to-failure hypothesis and the Multiple Critical Periods Hypothesis. The study includes 118 Mandarin-speaking immigrants in the United States and 24 English native speakers as the baseline comparison group. Grammar knowledge is assessed by a grammaticality judgment task, and phonology proficiency is measured by rating the degree of foreign accent in participants' production of a standard paragraph. Results reveal that ultimate L2 attainment is a complex phenomenon involving factors related to both nature (e.g., AOA) and nurture (e.g., L2 input). Although the AOA effect is robust for both linguistic dimensions, it is not deterministic or absolute for the grammar dimension. Post-pubescent learners are not doomed to fail in learning L2 grammar. The findings diverge for phonology and suggest strong age constraints on the production of L2 phonology: even an early start does not guarantee native-like outcome. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]