Question: what percent of the mass of the human body is made up of muscle?
Answer: Well, that depends on several factors including age and sex. For men aged 18-40, a normal body mass percentage would be in the region of 33.4 to 39.4%; between ages 41-60 we're looking at roughly 33.2% to 39.2%; and for the over 60s, as muscle mass fades naturally with age, I would expect to see ranges of 33% to 38.7%.