Mean and Maximum Life Span
The classical answer from experiments with mice is that "Exercise can increase mean life span but only calorie restriction increases maximum life span."
What this means is that if you put a treadmill in the cages of 100 mice, the average mouse will live a lot longer than mice that don't have a chance to exercise. But the last surviving mouse with exercise will live no longer than the last surviving mouse raised without any exercise. Apparently, there are a few mice endowed in such a way that they get the full benefit of exercise whether they exercise or whether they don't.
This is a strange result to understand, because in lab studies all the mice are genetically identical, or nearly so. They come from standard, inbred strains that come close to being like clones, or identical twins. They are also raised in identical cages at identical temperatures, fed the same food, etc. Why should they be different at all? We don't know. But typically they don't all die at the same time. Far from it. Typically, the spread in life span might be about 10% in either direction, for example an average of 750 days with a standard deviation of 75 days up or down, for a range 675 – 825 days. If the mice exercise, then the range might be compressed, say from 775 – 825 days.
Of course, best is to increase both mean and max life span.
But of the two, increasing mean without max is clearly an improvement...
...while increasing max without mean must involve a lot of extra early deaths.
The fact that we don't understand the variation makes it difficult to interpret changes in the variation.
The classical way to speak about this phenomenon is to say that "Only when the maximum life span increases can we truly say that aging is slowed down." For me, this just adds interpretive baggage to the basic result. If exercise has the power not just to decrease mortality risk, but also to delay the decline with age in health and vitality, that to me justifies the claim that aging has been forestalled.
I figure that I'm more likely to be one of the mice in the middle than the lucky one who gets the benefit of exercise without exercising, and so are most of my readers. For me, it's the mean life span that counts more than the max life span.