My understanding is that attributes will fluctuate based on use -- so an active person will maintain physical attributes while a sedentary person will lose them. This will actually be reflected in the appearance of the characters -- an active person will be muscular; an inactive one may even be obese.
There will be and should be limits on physical ability based on both age and genetics -- and I think they are implementing that. An active old person will be as physically fit as possible, but still not as fit as they were (or could have been) in their early adulthood.
That said, even today, not all old people can scale mountains, even if they remain as active as they can. But, I hope that the designers build that possiblity into the genetic rage of characters.