Generally around when you're too frail to safely stop unexpectedly. Or probably a little after that, just like how people keep driving their cars a bit too long

Generally around when you're too frail to safely stop unexpectedly. Or probably a little after that, just like how people keep driving their cars a bit too long. As an anecdote, my father has trouble walking nowadays - I think getting him to walk one km is ambitious. He does still use an electric bicycle though.