There are a lot of factors in how long you stay fit. My uncle Tim is only 11 years older than me. Though a true master craftsman, he chooses to cut firewood for a living at 83 yrs. old. Every September, after cutting wood through the summer from 6am--noon, he weighs exactly what he did in high school, 134 pounds. Needless to say he can still walk up the pheasants and anything else he wants to do. I wouldn't even blink if he wanted to do a mountain hunt this year--I know he could do it.
An old friend, Bill Crawford in Ft. Worth was going on what turned out to be his last mule deer hunt in Colorado. He always went in the rut, which meant lots of snow, and hunted on foot. I was a lot younger then and remember saying to myself, "man, he needs to rethink going out there alone." Now, I am the age he was then (72) and I don't think I am finished.