Aging affects immune function, increasing an organism's vulnerability to infectious and neoplastic diseases, however efforts in medical research have focused on adaptive immunity, leaving the role of innate immunity largely open for investigation. Echinoderms are an excellent model for studying innate immunity as invertebrates lacking adaptive immunity and as deuterosomes being more closely related to humans than any other invertebrate group. A survey of the immune gene repertoire encoded by the sea uchin genome reveals enormous and unprecedented complexity and it has been hypothesized that the robust innate immune system may play a role in both the longevity and resistance to disease that these animals exhibit. To begin to test this hypothesis, the objectives of this study were to investigate the complexity and function of the sea urchin innate immune system with respect to age. Coelomocytes are the immune cells, found in coelomic fluid that are highly variable in size, shape, and function. There are currently four main accepted groups; macrophages, red and white spherules, and vibratile cells. The composition of coelomocytes from the sea urchin Lytechinus variegatus was investigated using microscopy and flow cytometry and coelomocyte function was evaluated by the level of reactive oxygen species (ROS) detected using 2',7'-Dichlorofluorescin diacetate. The results showed a high level of variability in morphology and concentration of coelomocytes. A comparison of total cells, red cells, and colorless cells showed that smaller urchins had generally higher cell concentrations than large urchins and higher variability in the proportion of red spherule cells. Reactive oxygen species were detected in the coelomocytes of small and large urchins with the larger urchins showing a trend to produce more ROS. This suggests that immune function is maintained (and perhaps enhanced) with age in sea urchins. The trend of higher ROS production in older urchins is particularly interesting considering they tend to have lower coelomocyte concentrations and may have important implications for the maintenance of innate immunity with age. Given the close genetic relationship between sea urchins and humans, investigating the mechanisms by which immune function is maintained with age could possibly have larger implications for improving human health.