If you become a little less active and gain five pounds a year, that is not aging graceful. After a couple decades of that you’ll have 100 extra pounds of fat that may lead to metabolic syndrome - hyperinsulinemia, dyslipidemia, hypertension, hyperglycemia, and abdominal obesity. Your joints hurt, your feet hurt, and your breathing and walking become labored. Muscles weaken, and energy producing mitochondria go into an advanced state of disrepair. All this leads to more inactivity and eventually a heart attack or stroke.
From this study, Metabolic consequences of physical inactivity: