In ectotherms, age-specific traits related to heat absorption, such as body size or body color, possibly influence the regulation in body temperature responsive to external air temperature. However, this possibility has little been studied well yet, despite its importance in terms of their habitat utilization in the field. Here, using field and laboratory data, I evaluated the age-specific difference in individual response of some characteristics, activity rate and body temperature, under several air temperature conditions in the skink Plestiodon japonicas, whose color changes from black to brown with growth. First, I quantified the effect of air temperature on the rate and distance of walk. Second, I compared the difference in heat absorption between different age groups. Finally, I tested the effect of body color on heat absorption by comparing different-colored but equal-sized individuals. On high air temperature, young walk got shorter distance, while adult walk did not. Youngs also had the higher rate of heat absorption than adults. However, the effect of body color on heat absorption was not significant. From these results, I concluded that the age-specific body size is responsible for the different response of body temperature to the air temperature related to the different heat absorption rate in P. japonicas.