Cumulative distribution of the ratio of the Cepheid age t_exp(m) as expected from its present mass to its physical age t_phys (i.e. the time since its formation) for all Cepheids which are located in star clusters. Cluster metallicity is shown by colour. The grey regions indicate the cases where t_exp(m) differs by more than 40% from t_phys. These cases, whose fraction among all Cepheids is denoted p_mma, are likely to be detected by observers as Cepheids of discrepant ages.