In short
Sarcopenia is the progressive, age-related loss of skeletal muscle mass, strength, and physical function, distinct from simple muscle loss from disuse in that it is driven by a combination of biological aging processes.
Muscle mass and strength typically peak in early adulthood and begin a gradual decline around the fourth or fifth decade of life, accelerating further after roughly age 60–70, driven by a combination of factors including declining anabolic hormone levels, chronic low-grade inflammation, reduced physical activity, motor neuron loss affecting muscle fiber innervation, and decline in muscle satellite cell (stem cell) function and number.
Diagnostic criteria have been formalized by working groups including the European Working Group on Sarcopenia in Older People (EWGSOP2, 2018 revision), which defines sarcopenia based on low muscle strength (commonly measured by grip strength) as the primary indicator, confirmed by low muscle quantity or quality (often measured by DEXA-derived appendicular lean mass), with poor physical performance (such as slow gait speed) indicating severe sarcopenia.
Resistance exercise training has the strongest evidence base among interventions for preventing and treating sarcopenia, with numerous randomized trials showing it increases muscle mass and strength even in adults in their 80s and 90s, and adequate dietary protein intake (generally cited in geriatric nutrition guidelines as somewhat higher than the general adult RDA) supports this effect. Pharmacological approaches, including testosterone replacement in men with diagnosed low testosterone and some GLP-1 receptor agonist research examining lean mass preservation during weight loss, are areas of active study but are not a substitute for resistance training as the primary evidence-based intervention.