The secular trend of growth in height suddenly slowed for Americans born after the middle of the 20th century, and the health and human capital of these cohorts as adults appears to have declined, or at least stagnated, more broadly. This paper presents evidence that the physical growth of these unhealthy cohorts was particularly stunted during adolescence. Using data from NHANES and its predecessors, I show that males born in the 1960s were the same height in childhood as those born a decade earlier, but then fell behind and were half an inch shorter in adolescence. By adulthood, the heights of the two cohorts were nearly identical. This suggests that males born in the 1960s had a later or smaller adolescent growth spurt than those born a decade earlier. Similar patterns are not evident in the height of females; however, females born in the 1960s experienced menarche later than those born a decade earlier. The delayed puberty of cohorts born in the 1960s appears to be a short-term blip in a long-run trend towards earlier puberty. The findings strongly suggest that something had already gone wrong by at least adolescence for American cohorts born after mid-century.