In this study, we used the lifespan theories of selection optimization and compensation (SOC) and the socio-emotional selectivity theory (SST) to integrate the disengagement and activity perspectives on aging and to explain intra-individual changes in work motivation. A two-wave longitudinal survey study was conducted among 600 Dutch university employees to test relations between intra-individual changes in subjective general health, future time perspective (FTP), and generativity and development motives. In line with the two lifespan theories, two processes were found. First, consistent with the disengagement perspective on aging, losses in subjective general health negatively predicted open-ended FTP, and open-ended FTP was positively associated with an increase in growth motives. Secondly, consistent with the activity perspective on aging, losses in subjective general health positively predicted limited FTP, and limited FTP was positively associated with an increase in generativity motives. In addition, initial open-ended FTP predicted future open-ended FTP through a change in subjective general health. Finally, these relations were found to differ across age groups, and thus over the lifespan. More specifically, losses in subjective general health and increases in limited FTP particularly result in lower development motives and higher generativity motives among middle-aged workers, indicating that interventions aimed at preventing losses in health are probably most effective for maintaining development motives in this group.