A young age at arrival is believed to be an important predictor of adult immigrant achievement, but there is no consensus on what age(s) at arrival is pivotal/crucial/critical. The 2000 census reports exact years of arrival and age providing us the opportunity to test different formulations for age-at-arrival effects for several different socioeconomic outcomes. We focus on the experiences of Mexican immigrants to the U.S. in this study. Our results indicate That the effect of early arrival is much greater for English proficiency than other outcomes and bears significantly on most, not all, attainments. There is little evidence at any age of a sharp discontinuity demarcating a 1.5 generation from older immigrants and, in fact, a series of classifications or a continuous measurement of age at arrival may be preferred in some cases. Guidelines are offered for the most appropriate formulation of age at arrival under different contexts.