Shit, even going from a semi active lifestyle to a 9-5 office job will add a belly if you’re not hitting the gym.
Only if you keep eating as you did before. People don’t change their diets as they get older. They seem unaware / not caring that the belly will come otherwise.
I think it’s partly just being OK with getting fat. If you are not attractive anymore anyway because of age, may as well enjoy whatever food you like. And if you are in a relationship, both partners are probably OK with looking not so great anymore as they age.