Age 69, as of a few days ago, and I ride in positions 1 and 2 almost exclusively. Breaking my left elbow (for the second time) at the beginning of my growth spurt at age 12 or so resulted in a deformed bone structure, which, in turn, ensures hand numbness whenever I ride a bike for more than fifteen minutes in any of the other positions. I used to put up with the numbness, but then aero bars were invented. Using aero bars on all of my bikes means that I can ride all day without hand numbness.

Stretching always seemed a waste of time for me; same with the currently fashionable "core" exercises. Echoing CarbonFiberBoy: putting in the miles is pretty much all I need to do to ride comfortably in my preferred positions.