We grow or we atrophy. It's really a matter of choice. Keep moving forward by walking, jogging, and/or running and we'll keep that ability far longer than If we don't. Keep going up stairs and we'll keep that skill. Keep opening up tough jars and we'll stay strong. Carry our own groceries and we'll keep that strength. Use our glutes, quads, hamstrings, pecs, lats, Delts, bi's, tri's, abs, and low back and those muscles will stay strong and taught. Neglect them and they won't. Get the heart rate up on a regular basis and the heart will work for you. Don't exercise it and it will shrink up and stop. Choice not fate. Use the brain: read, pay your own bills, learn skills, study computers. If you do your brain still stay strong. We are so much more adaptable than most people realize. But we have to turn the key. Our body want start itself.