This study examined the age-specific pattern of effects of welfare policies on child achievement. Drawing from 7 random-assignment welfare and antipoverty evaluations that provided more than 30,000 observations of children's achievement, this study found that times of developmental transition are the only periods sensitive to the changes in families brought about by these policies. More specifically, small positive effects of welfare and antipoverty policies were found for children making the transition into middle childhood, and small negative effects of these same policies were found for children making the transition out of middle childhood and into early adolescence. Effects were robust across various program groupings and could not be attributed to family characteristics that differ for children of different ages. This research informs the understanding of how changes in employment and income for low-income parents affect development across childhood.