Peak muscle mass happens around, between the ages of 20 and 30. And then after that, as you start to get into your 40s and 50s, you lose about 8% of muscle mass per decade. Once you get into your 70s, 15% of muscle mass per decade. So most people, by the time they're 70 or 80-years-old, only have about 60-80% of the muscle mass they had when they were 30.