The great secret that all old people share is that you really haven't changed in 70 or 80 years. Your body changes, but you don't change at all.
That is what learning is. You suddenly understand something you've understood all your life, but in a new way.
All sanity depends on this: that it should be a delight to feel heat strike the skin, a delight to stand upright, knowing the bones are moving easily under the flesh.