Both aging processes and psychological stress affect the immune system: Each can dysregulate immune function with a potentially substantial impact on physical health. Worse, the effects of stress and age are interactive. Psychological stress can both mimic and exacerbate the effects of aging, with older adults often showing greater immunological impairment to stress than younger adults. In addition, stressful experiences very early in life can alter the responsiveness of the nervous system and immune system. We review the unique impact of aging and stress on immune function, followed by evidence of interactions between age and stress. Further, we suggest that prenatal or early life stress may increase the likelihood of maladaptive immune responses to stress in late life. An understanding of the interactive effects of stress and age is critical to efforts to determine underlying mechanisms, clarify the directionality of effects, and develop effective interventions in early and late life.