We developed a simple back-calculation model that includes the age effect in which otolith size increases continuously during no-growth periods. To evaluate the validity of our proposed model, we back-calculated the past fish lengths and growth rates using the new model and seven traditional back-calculation models (scale-proportional hypothesis (SPH), body-proportional hypothesis (BPH), Fraser–Lee, biological intercept, nonlinear SPH, nonlinear BPH, and modified Fry) using the otoliths of individually tag-recaptured white-spotted char (Salvelinus leucomaenis). The estimated fish lengths corresponded well to observed fish lengths for simple traditional (SPH, BPH, and Fraser–Lee) and the new models. However, the back-calculated growth rates did not correspond to observed growth rates except for the new model. All previous models had considerable bias; growth rates of slow-growing fish were overestimated. As our model incorporating the age effect did not show such bias, this bias would be attributed to the age effect. Our proposed back-calculation model incorporating the age effect should be useful to estimate past growth rates at the individual level.