Muscle mass begins to decline starting in a person's 30s if left unaddressed.
"And that process actually starts earlier than most people think. You're in your 30s, your muscle mass, if you leave it alone— Starting to decline." (said at 0:17:18)
A common clinical and physiological consensus is that peak muscle mass is reached in early adulthood (around the third decade of life) and begins a gradual decline of approximately 3% to 8% per decade thereafter in sedentary individuals. However, population-based cohort studies measuring body composition across the lifespan demonstrate more nuance: peak skeletal muscle mass is frequently maintained through the 30s and early 40s, with significant, progressive loss of total and appendicular lean mass typically beginning between the fifth and sixth decades of life (ages 40–59), whereas measures of neuromuscular power and strength tend to decline earlier than raw muscle mass.
- context: The Copenhagen Sarcopenia Study: lean mass, strength, power, and physical function in a Da… (Journal of cachexia, sarcopenia and muscle 2019)
"While the power-based measurements (LEP and 30 s STS) started to decline already at age +50 years, less power-based parameters (GS and HGS) and lean mass characteristics (TLM, ALM, and ALM/h 2 ) remained unaltered until after the age of +70 years." (abstract, conclusions, passage verified)
pubmedfull study (doi) - context: Age- and sex-specific normative values for muscle mass parameters in 18,625 Brazilian adul… (Frontiers in public health 2023)
"In women, muscle mass peaked between the ages of 40-49 before gradually declining at an average rate of 5.7% per decade from the sixth decade of life onwards. ASM reached its peak earlier, during the third decade of life, and started to decline later, from 50 to 59 years. In contrast, absolute and ASM peaked at 40-49 years and declined from the sixth decade of life in men." (abstract, results, passage verified)
pubmedfull study (doi)