Sociologists typically assume that immigrants' acquisition of English as a second language follows the opportunities and motivations to become proficient in English, while many linguists argue that second language acquisition may be governed by maturational constraints, possibly biologically based, that are tied to the age at onset of language learning. In this article, I use U.S. census data to investigate the relationship between age at onset of second language learning and levels of English language proficiency among foreign-born adults in the United States. The overarching conclusion is that proficiency in a second language among adults is strongly related to age at immigration. Part of that relationship is attributable to social and demographic considerations tied to age at entry into a new country, and part may be attributable to maturational constraints. Many colleagues, spread across several disciplines, provided valuable comments on this research. Among them were Cynthia Fisher, Nancy Garrett, Susan Gonzo, John Hagan, Charles Hirschman, Tim F. Liao, Michael Long, John Mardeen, Madonna Harrington Meyer, C. Gray Swicegood, and Jack Veugeler.