48, but who's counting? I guess I am because I'm due for a physical and getting ready (psychologically) for all of those recommended "Age 50" doctor appointments. Like your 50,000 mile car appointment that covers everything and changes all of the fluids and shocks you with the cost and will hopefully tell you that everything is good.
I recently realized that my aunt had both her knees replaced at 48, with a non-athletic background and no prior injuries. Just overweight and no exercise. She's still around at 75 though I think the warranty on those knees is up.