Purpose: The aim of our work was to develop an approach to account for the impact of age at exposure on acute radiation lethality risk for the purpose of improving casualty estimation tools when applied to a diverse population. Materials and Methods: Age-dependent radiation lethality data were collected from published animal studies. The 50% lethal dose responses (LD50) were extracted, grouped according to developmental stages in humans and average LD50 values calculated for select age categories. Dose modification factors (DMFs) were developed by dividing LD50 values of non-young, adult groups to the reference adult category within each study. DMFs were combined across each age group to provide a DMF for each age category. Results: Data from 12 studies with age-dependent LD50 values from 5 species (>21,000 animals) demonstrate increased sensitivity to acute radiation in elderly and young animals compared to young adults. DMFs were developed for infant (0.80), juvenile (0.86), late adult (0.86), and elderly (0.71) populations. Conclusions: Animal and human data support increased radiosensitivity in infants, juveniles, and aging adults. DMFs provide a mechanism to account for age-dependent variability in health effects models and to determine the impact of age on casualty estimates.