Why do old people fall and then are unable to get up? I don't see people falling on a regular basis, even at the old folks home.
-Penguin
Take your pick: balance issues, frail bones (osteoporosis), joint mobility (arthritis), loss of muscle tone, etc., etc., etc...
The truest words my ninety-one year old Mother-in-law ever said to me were, "This getting old business is not for the faint of heart!"