In this study we use the concept of the marginal migrant to develop a theoretical model of the age composition of US immigration. Two age classes of immigrants are distinguished, 20-34 and 50 and over, along with sex and entry class (numerically restricted, numerically exempt). Annual data (1972-1991) for 109 source countries are pooled, and the Hausman-Taylor instrumental variable technique is employed to estimate regressions that satisfy adding-up restrictions. Along with measures of differential economic opportunities, migration costs, and US institutional controls, we incorporate into the analysis a unique set of variables relating to social programs in source countries. Such programs prove to be significant determinants of the age composition of US immigrants. (C) 2002 Elsevier Science B.V. All rights reserved.