There is growing evidence that male age affects sperm production and quality, with reductions in sperm traits observed in both vertebrates and invertebrates, suggesting that this is a general trend. However, many studies do not observe a decline with male age, making the issue of male age-related changes in sperm fitness equivocal and contentious. Two key confounding factors in prior studies has been the lack of standardization of male mating history and female age. For example, males that mate often, or experience high rates of sperm competition, may replenish their sperm frequently and thus have better fertilization success, regardless of age. Likewise, it is well known that female reproductive success alters with age. To better understand age-related declines in male fertility, we recently completed a 28-month longitudinal, cohort based aging study, using zebrafish (Danio rerio) as a model. We investigated if sperm traits alter with age and mating frequency and how age-dependent patterns of decline in sperm fitness and mating frequency affect male reproductive success and offspring fitness, while controlling for female age. Our work demonstrates that sperm performance declines with increasing male age, but also suggests that survival is higher in offspring of older males than younger males. We have also been conducting meta-analyses to quantify the effect of male age on sperm traits in both human and non-human studies. Overall, our work provides improved knowledge of age-related fertility declines and the mechanisms responsible for the decline, the majority of which are unexplored outside of humans.