https://www.builtlean.com/2014/03/03/70-year-old-man-muscle/Well you lose muscle due to atrophy, not just getting old. Muscle fibers do srhink if they are not stressed on a regular basis over time, no matter your age (i.e. have you seen a teenagers arm after a cast is taken off after 6 weeks?)
People who train their muscles while they age still have their muscles so it is not just causation.
