The amount of muscle mass a person has depends on many factors: genes, age, body size, diet, and level of previous physical activity. Muscle mass generally has the potential to be the greatest during young adulthood because of the hormones present at that time. Taking advantage of this stage is very important for world-class athletes, who may actually reach their peak muscle mass. But most of us do not train extensively and instead spend much of our lives with muscle mass far below what would be possible with careful diet and regular exercise.
Research has clearly shown that people of any age can increase their muscle mass–even people over age 80. Any increase in physical activity has the potential to do this. Resistance exercise (like weight lifting) has a greater effect on building muscle mass, while aerobic exercise tends to build endurance. But any kind of exercise increases both.
Athletes who have spent much of their lives in top physical condition should expect a very gradual decrease in muscle mass with age, despite continued training. Those who are not yet in peak physical condition still have significant potential to increase muscle mass to the highest level obtainable for their age. Resistance exercise works particularly well this way. If you feel that you are losing muscle mass or strength at a faster-than-normal rate, you should see your doctor to make sure this is not due to a medical problem.