Age distribution of the cases with novel coronavirus disease (COVID-19) is rather different from that of influenza. We examined the age distribution of COVID-19 cases in Japan from January to March, 2020. Children are less likely to be diagnosed as cases, and moreover, the risk of disease given exposure among children appears to be low. Both the overall risk and the conditional risk of disease given exposure are likely to be the highest among adults aged from 50-69 years. The most plausible explanation that we believe is immune imprinting to a similar virus among adults.