Background and Objective: As part of the enlightenment campaign for COPD and for the introduction of an easy method to assess aging phenomena in respiratory function, the indicator of lung age has been suggested by the JRS Lung-Age-Spread-Promotion Secretariat (original method). In this original method, the lung age was estimated by counting back the regression formula predicting the reference value of FEV₁. Since the normal value of FEV₁ at a given age is not unique and exists within a certain range defined as the 95% confidence limit, the backward value of lung age calculated with the original method includes various statistical and physiological problems. In the present study, I proposed a novel method allowing estimation of lung age, in which the problems related to the original method were significantly overcome. Results and Conclusions: Since the 95% confidence limit of FEV₁ was not considered in the original method, the lung age of a person with high FEV₁ beyond the upper end of the 95% confidence limit would result in the subject being classified as remarkably young (sometimes, the calculated lung age is below zero), while that of a person with reduced FEV₁ below the lower end of the 95% confidence limit would be estimated as being very elderly (sometimes, the calculated age is over 100). On the other hand, the novel method reasonably deliberates the 95% confidence limit, leading to the conclusion that it could be applied for estimating the lung age of persons having a wide range of FEV₁.