Obviously there isn’t a sharp age cut-off, but many elderly people start having problems with vision, hand-eye coordination, reaction time, joints, memory, ...
Often elderly people’s driving skill deteriorates before they realize it or they are unwilling to accept the loss of mobility, putting themselves and other people on the road at risk.
When elderly people do accept that they won’t drive anymore, in a car centric society they are suddenly dramatically hindered in their ability to lead a normal life. They become much more dependent on constant care and attention, and are often forced to move into a dedicated retirement home, further alienating them from their life-long communities of institutions and friends.
Places where it is possible to walk to the store, walk to visit friends, walk to the subway station, walk to coffeeshops and restaurants, walk to concerts and museums, etc. are much better for the elderly than suburban sprawl. People can remain independent much longer, because there is more available community infrastructure nearby.
Additionally, maintaining a routine with a decent amount of daily walking is very beneficial to people’s health.