For more along these lines, check out the recent NYT article, What to do now to feel better at 100:
Muscle strength also declines with age, even in the absence of a muscular disease. Most people (bodybuilders excluded) achieve peak muscle strength between 20 and 30, with variations depending on the muscle group. After that, strength slowly declines, eventually resulting in telling symptoms of muscle weakness, like falling, and difficulty with essential daily tasks, like getting up from a chair or in and out of the tub.
Most otherwise healthy people do not become incapacitated by lost muscle strength until they are 80 or 90. But thanks to advances in medicine and overall living conditions, many more people are reaching those ages, Dr. Lachs writes: “Today millions of people have survived long enough to keep a date with immobility.”
The good news is that the age of immobility can be modified. As life expectancy rises and more people live to celebrate their 100th birthday, postponing the time when physical independence can no longer be maintained is a goal worth striving for.