Socioemotional selectivity theory and models of lifespan development of resources suggest that older workers may particularly benefit from social resources so as to maintain their well-being throughout their work-life span. However, the age-differential effects of social support at work have been rarely investigated. We hypothesised that age moderates the effects of colleagues' and supervisors' social support on mental strain, with strongest effects for older workers. A two-wave complete panel design (six months' time lag) was used. Self-reports from N = 334 nurses (age: 21-63 years) were gathered with established questionnaires: strain was measured by means of the irritation scale; social support from colleagues and supervisors by using a German adaptation of the social support scales. We proved factorial validity and measurement invariance across time points (CFA) and computed path models (SEM). As expected, age moderated the negative longitudinal effects of colleagues' social support on mental strain. Older nurses (≥45 years) benefited the most from colleagues' social support. However, mental strain in older nurses was associated with reduced social support from colleagues. Surprisingly with middle-aged nurses (35-44 years) an increase in colleagues' social support resulted in higher mental strain. No effects for supervisor support were observed. Results indicate that social support by colleagues is an important resource for older workers, but older workers are less likely to receive social support when mental strain is present. Moreover, the timing of social support across the work lifespan seems to be critical, as it might have detrimental effects in middle-aged workers.