I think who can do what at what age varies a lot, it isn't that there are certain jobs that any 71 year old could do, though some are more likely to be manageable than others.
Having said that, I'm currently watching my fit and active DH less and less able to carry heavy objects or to do fiddly DIY jobs without injuring himself. He's no less active than he ever was, so it is just age having its effect. And I've had to enormously reduce my working hours because concentrating for hours a time is exhausting nowadays and I start making mistakes much sooner than I used to. Again, cutting down was in response to this not a cause of it; I imagined being able to work full time much longer than is turning out to be possible.
Like so many things nowadays, the prospect of pension starting in the seventies is frightening.