Ageing and muscle mass
After the age of 50 the average adult loses 1% of muscle mass every year. As we lose muscle mass we tend to do less because we feel weaker, this in turn accelerates the problem through inactivity. Trips and falls become more common due to simple things like not picking your feet up quite as much as you used to.
There is an old saying use it or lose it, and its true.
Muscle mass is easily recoverable all you have to do is exercise regularly and lift light weights. One of the best types of exercises is using your own body weight to strengthen your body plus some light kettle bell routines. Bottom line is if you practice stretching, balance and form work combined with strength training you can be fitter at 75 than you were at 50. People who practice traditional soft martial arts in the far east are the probably the fittest older generation in modern times.