  It is well established that the Hubble residuals of type Ia supernovae (SNe
Ia) show the luminosity step with respect to their host galaxy stellar masses.
This `mass-step' is taken as an additional correction factor for the SN Ia
luminosity standardization. Here we investigate the root cause of the mass-step
and propose that the bimodal nature of the host $age$ distribution is
responsible for the step. In particular, by using the empirical $nonlinear$
mass-to-age relation of local galaxies, we convert the mass function of SN Ia
hosts to their age distribution. We find that the age distribution shows clear
bimodality: a younger ($<$ 6 Gyr) group with lower mass ($\sim 10^{9.5}{\rm
M}_{\rm sun}$) and an older ($>$ 6 Gyr) group with higher mass ($\sim
10^{10.5}{\rm M}_{\rm sun}$). On the Hubble residual versus host mass plane,
the two groups create the mass-step at $\sim 10^{10}{\rm M}_{\rm sun}$. This
leads us to conclude that the host galaxy mass-step can be attributed to the
bimodal age distribution in relation to a nonlinear relation between galaxy
mass and age. We suggest that the mass-step is another manifestation of the old
`red sequence' and the young `blue cloud' observed in the galactic
color--magnitude diagram.
