Why? They have needs, desires...
Naturally women go through menopause, and men get ED among other health aliments that affect libido, desire, erection, not to mention the physical strength to perform the act itself.
If you grow old enough to be 70, and you happen to be married, and your wife is down, i have no objection.
But to go on the prowl looking to hook up when you're that old, lol.