I occasionally have speculated that our aging society would lead to increased transit ridership, as seniors lost the ability to drive. But I recently discovered that seniors are actually less likely to use public transit than the general public. One study by the American Public Transit Association showed that 6.7% of transit riders are over 65 (as opposed to 12.4% of all Americans).(1) The oldest Americans are even more underrepresented on America's buses and trains: only 1.5% of transit riders are over 80, about half their share of the population (2). The only other age group that is underrepresented on public transit is Americans under 18.
Why might older Americans be more car-dependent than younger Americans? It could be argued that older Americans are "locked into" car-dependent sprawl, having bought houses many years ago. But 55-64 year olds are probably just as "locked in" to their houses, but according to APTA's on-board survey of transit riders are actually slightly more likely to use transit than the general population (though less so than 20- and 30- somethings).(3) So this explanation is unlikely to be the best one.
A more likely reason is that older Americans are less likely to work, giving them less incentive to travel generally. But why aren't retirees who do travel abandoning driving for transit in large numbers?
Now that I have a not-very-mobile 89-year-old father, I can suggest one possible explanation: if your body is sufficiently troubled that you can't drive, it is often sufficiently troubled that you can't walk very far (or very safely, given the high level of harm from falls), which in turn keeps you from walking to a bus.
(2) Id. at 38.
(3) Id. at 37. However, the evidence as to this age group is more ambiguous; a national household survey showed that 41-60 year olds were slightly underrepresented on transit, though less so than seniors. Id. at 38.