never.. as long as you are physically healthy enough for it. ;)
When you die...that's when you're too old.
I would doubt you are never to old to have sex.
Hopefully never...although I will admit that I gross myself out envisioning my husband and myself over age 70 gettin' it on, lol.
By the time we actually get to that age, though, I'm sure we won't gross ourselves out, seeing as we'll have had over 20 years from now to get used to our aging process and appearances, haha.
Apparently never...look at Hugh Hefner! LOL!
?? hopefully never, LoL ;)