This study pays attention to the necessity for discussion of determinants in the utilization of flexible benefit plans, and examines whether age diversity increases the utilization of flexible benefit plans or not from the intra-organizational ecology perspective. In addition, this study considers the inflow of workforce from the external labor market (experienced employees) and environmental complexity as the critical moderators influencing the relationship between age diversity and the utilization of flexible benefit plans. In other words, this study predicts that these moderators will strengthen the main effect. The results of longitudinal analyses with HCCP datasets showed that age diversity increased the utilization of flexible benefit plans, as predicted. However, the moderating effect of the inflow of workforce from the external labor market was not statistically significant, and the moderating effect of environmental complexity rather demonstrated the opposite direction compared to the hypothesis. More specifically, when the degree of environmental complexity is low, the positive relationship between age diversity and the utilization of flexible benefit plans became more pronounced. In the discussion part, this study provides the possible explanations for unsupported hypotheses, and suggests the implications as well as limitations for future research.