This paper argues that the changing age composition of the nation's population has contributed significantly to recent major shifts in U.S. interregional migration. During the 1970s, the large baby-boom generation came of age, and the net outflow of population from the Northeast and Midwest regions to the South and West regions increased precipitously, with net migration between these regions reaching a level in 1975-80 approximately three times greater than in 1955-60 or 1965-70. To clarify the different ways that age-composition change has influenced such recent trends, three hypotheses are explored using a spatial shift-share decomposition model. The model first demonstrates that the significantly larger cohorts of the baby boom generation supplied a basis for the 1970s migration shifts, but the geography of the baby boom does not, by itself, provide a sufficient explanation. Secondly, it discloses notable changes in age-specific mobility rates consistent with economic theories about the repercussions of disparities in the sizes of successive generations. Although the implications for regional population growth or decline of these mobility effects are significant, particularly in suggesting a reason for the slowing of movement out of the South, the paper argues that the largest contribution of age-composition change is expressed in a third way, termed a geographic distribution effect. I suggest that heightened demographic effectiveness of channels of population interchange has resulted from the differential abilities of regional labor markets to accommodate the labor-supply pressure caused by members of the baby boom generation seeking entry-level jobs.