For the age count between citing and cited papers, the difference in the year of publication is widely used. In some cases, the frequency obtained is inaccurate because of age bias. This article describes a probabilistic method for adjusting the raw frequency, and its application to the impact factor and immediacy index in journal evaluation. The main procedure is that 1/8 of the raw frequency at any given age is assigned to the previous age and to the following age, respectively. Adjusting the frequency over all ages of use, the curve representing the age distribution becomes smoother.