Just as fun and helpful is to join a climbing gym if one is nearby. I also find that regularly babysitting the little grandkids keeps one in shape! You are always squatting on the floor to get to their level, lifting and chasing after them.

I am convinced that flexibility/agility problems are a major factor in older backpackers quitting. At one point it is too difficult to live on the ground, get in and out of the tent and get up off the sleeping pad.