I found a study of lean mass in 1,200 average men (not specifically athletes) aged from 30 to 80 y/o.
"Lean mass" here is body weight, less fat mass and bone mass. It looks like lean mass stayed pretty constant from 30 to 59 y/o, then declined from 60 y/o on. The ratio of lean mass to height only declines a little throughout the period, reflecting getting shorter after about age 60 y/o. Grip strength declines steadily during the period, suggesting a steady decline in muscle tone.
Statistically speaking this is not a great study - cross-sectional not time series, for one thing - but still interesting. Suggests that if you don't add fat mass, there is not a inherent reason to get heavier as you age from 30 y/o on. Makes my avoirdupois even less tolerable.
BioMed Central | Full text | Lean mass, muscle strength, and physical function in a diverse population of men: a population-based cross-sectional study