Oxidative stress and chronic inflammation have been implicated in the testicular aging process. Different types and moderate-intensity of regular exercise may reduce age-related physiological dysfunction associated with inflammation and oxidative stress, but such effects of moderate-intensity of exercise over different phases of life in testes have not been reported. In this study, male SAMP8 mice, a senescence-accelerated strain, were maintained as sedentary (sed) or subjected to daily 15-min periods of swimming exercise between ages of 2–7 months (lifelong), 2–4 months (earlier) or 5–7 months (late). Age-related changes, including serum testosterone levels and biomarkers of inflammation and oxidative stress were analyzed at the end of the experiment. All exercise groups showed significantly greater serum testosterone levels and decreased age-related inflammation and oxidative stress compared with the sedentary group. Exercise also increased expression and activity of the nuclear factor erythroid 2-related factor (Nrf2), a transcriptional regulator of the cellular anti-oxidant system, and decreased expression and activity of nuclear factor kappa beta (NF-κB), a mediator of inflammatory molecules, in the nucleus of testicular cells. However, lifelong and earlier groups generally showed significantly better protective effects than the late group against age-related physiological dysfunction in testes. Thus, lifelong exercise and earlier phase exercise were most effective in counteracting oxidative stress and inflammation and in preserving testes function through regulation of Nrf2 and NF-κB. These results advocate the benefits of lifelong exercise and emphasize a greater protection against male aging by instituting exercise earlier rather than late in life.