It totally depends on 1) how old I happen to be and, 2) what the lady I'm considering happens to look like. There is no definitive answer.
You young fucks here need to remember you'll be my age before you know it. At that stage, a sexy 60 year old holds quite a captivation. I regularly bang my wife who is 54 years old and still can fuck like she's 24....just not as often or as long (nor me I hate to admit).