Obviously I believe that there's no age at which to stop having sex. But I have heard from some people in their 20's that the idea of people over the age of 50 is kind of repellent.
I was talking to a friend's son after my friend was in a car accident where his pelvis was crushed. The son told me that the doctors said there was a chance of impotence after healing, "not that that will matter to Dad." I expressed surprise because "Dad" was 54 and someone I happen to know is quite sexual. I told the son that I thought his dad would be quite concerned - he's still young and even dating. This 25 year old man expressed shock & surprise as he thought that once someone hits 50, sex is no longer an issue.
I've heard this same view espoused by a few others in their 20s. Perhaps it's because from the age of 20, the 40s, 50s and beyond seem so far away. Maybe they don't realize yet how young one still feels at those ages. Maybe it's all those ED commercials that make it seem like erections are too difficult. I don't know, but it's kind of interesting.