Researchers often estimate models of early retirement that include a control for whether a worker is in a blue-collar job – basically assuming that less-physical white-collar work allows longer careers. But this assumption ignores the fact that even workers in white-collar occupations may find themselves relying on skills that have declined. This paper instead reviews the literature on aging and constructs a Susceptibility Index meant to reflect how susceptible an occupation is to declines in ability, regardless of whether the occupation relies on physical abilities (as blue-collar occupations do) or cognitive ones.