It hugely on body type and athletic history. I haven't done any real exercise in the last 5 years, and I'm easily 25 pounds above ideal body weight, but once upon a time I was a beast. I'm in my mid-40s and I just did 40 clean push-ups on the floor next to my computer without any trouble - and again, I haven't done any actual upper body training in literally years.

The trick is to have super short legs, very broad shoulders, a barrel chest, and a history of serious swimming and weight training. It helps to have the body proportions of a gorilla if push-ups are your metric. But if I had to run two miles without stopping I would literally drop dead.

Obviously these sorts of studies apply to the averages and not the outliers, but I didn't see any attempt in the methodology to control for body type.