WHEN SHOULD WE STOP HAVING SEX?

I was discussing this issue with a girlfriend, and we were wondering when should you stop having sex? I look at my mom who is 83 years old and I cannot imagine her still being sexually active, also I cannot picture anyone over the age 60 still having sex. Now I know a lot of people think as long as you are healthy you can still have it but I just cannot see myself at 65 trying to still do it. I know they have the blue pill for the men and will soon have something similar for the women. I just look at how your body looks at sixty and cannot see how I could be attracted to some old guy with everything hanging in the wrong place. Maybe when I reach that age maybe I will think differently, but for now I cannot see it. What do you think?