Using current methods of age determination, white-tailed deer (Odocoileus virginianus) from the same cohort are classified as 1.5-year-old deer when actual age may vary by 6 or more months. We measured mandibular tooth row length (mm) on mandible impressions of 41 (23 males and 18 females) known-age captive white-tailed deer to develop an aging model that would estimate age in days. To test the model, each month from 12 to 24 months, we measured dentitions of 34 (19 males, 15 females) known-age captive white-tailed deer. We found that mandibular tooth row length was a strong predictor of age for both males (r2 = 0.93; P ≤ 0.0001) and females (r2 = 0.92; P ≤ 0.0001). For males and females, respectively, the equations that predicted age (days) were ln (age) = −4.34 2.45 ln (mandibular tooth row length in mm) and ln (age) = −4.62 2.54 ln (mandibular tooth row length in mm). The models predicted >50% of test jaws for males from 14 to 24 months and >60% of test jaws for females from 16–24 months within 7 days of actual age. Peak accuracy occurred at 18 months for males and 19–21 months of age for females. These models complement current aging methods and could be used by wildlife biologists in the field to age hunter-harvested deer ≤24 months.