I have clients that still go on trips their DP 3 or 4 times a year and they are in their mid eighties. They probably run 4-5 hrs a day and just recently decided to have it washed and waxed for them instead of doing it themselves.
But age numbers really never matter, it's all about staying active and having work to do around your place, always have goals or project to keep you going.
I think the important part of this is to have projects that really motivate you that create a desire. Create something never done or doing something better than anything available on the market. Its a cool thing to be involved in challenges this way with my client. He's always got something going. It's amazing how smart he is about solving a problem on designing something.