Both plant senescence and plant response to ionizing radiation involve changes in gene expression and epigenetic profiles, that rely on the formation of reactive oxygen species. However, how the developmental stage of a plant affects its response to ionizing radiation has not been extensively studied. In this study, our experiments showed that exposure to low (10 Gy) and high (100 Gy) doses of ionizing radiation causes developmental delays in plants that may result in reduced biomass or even death of the organism. In particular, 20-day-old plants, which are in the process of transitioning to reproductive growth, showed a distinct response to irradiation compared to 10- or 30-day-old irradiated plants that affects the expression of DNA repair genes. Specifically, we found that the expression of mismatch repair genes was increased in 20-day-old plants, while RAD51 was increased in 10- and 30-day-old plants. Furthermore, we found increased expression of MET1, CMT3 and SUVH5 epigenetic regulators that paralleled decreased ONSEN transcript levels in 20-day-old irradiated plants. These findings suggest that plants exposed during early reproductive growth exhibit a tighter control over genome stability in response to ionizing irradiation compared to plants irradiated at other developmental stages.