While US wealth inequality has been rising, research and policy debates have paid little attention to the fact that wealth inequality contains two dimensions - inequality among individuals of the same birth-cohort and inequality between birth-cohorts. We disentangle these dimensions and show that increasing wealth inequality is to a considerable extent driven by between-cohort inequality. We further show that a significant part of this between-cohort inequality may be linked to optimizing saving behavior of individuals over their life cycle, thus not necessarily pointing to a need for government intervention.