The rapidity with which immigrant children learn the dominant language of their country of residence has important short-term and long-term consequences for their educational achievements and for their future. In this paper I use US Census data to model trajectories of English acquisition among foreign-born children living in Spanish- language households. The results show, as expected, that children's English proficiency increases with length of residence in the United States. However, the results also show a clear trend by age at arrival. The older children are when they arrive in the United States, the less rapid their progress in acquiring proficiency in English. [ABSTRACT FROM AUTHOR]