Starting at age 30, most people begin to lose strength and muscle mass. Between 30 and 50, the average person suffers a 10% loss of muscle.[i]
After that, muscle loss accelerates. Between 50 and 60, muscle loss averages 10-20%. After age 60, it speeds up even more. This is true for both men and women.
The amount of muscle loss over a lifetime varies greatly from one person to another.