I am 70 and my wife is 72. We still have sex about once a week
I know of couples in their eighties who still have sex. I know I certainly hope my husband and I will be enjoying each other sexually when we are in our eighties (and nineties too)!
my grandparents are in their mid 70's and still have sex. (with each other no one else is involved)
The second or third year of marriage.
I bet it depends a lot on how long they've been together. Two 60-year-olds can get married and be happy for years, but another couple that's been married 35 years already has too many issues (I think) to keep it going. I don't know anyone who still has sex after 30 years of marriage.
Meet my wife and me; married 39 years and have sex maybe two or three times a month. She looks great, and she feels great too...if you get my meaning! Long may it continue.
Hopefully to the bitter end.
Around 50 when they can no longer reproduce.