Metabolism may not change on average, but that doesn't mean it can't change or differ wildly between people.

Look, I know some people who obsess over their calories, eat perfectly, and exercise constantly, and they still barely hit their "goal" weight/fitness. And I know other same-age people who do no exercise and eat fast food for almost every meal who are still skinny.

Maybe age isn't necessarily the reason, but we can't pretend it isn't easier for some people to lose/maintain weight than others. There ARE differences in metabolism.