"There's no biologic reason to get fatter as you grow older," writes Clarence Bass is this excellent short explanation of what metabolism is and how, "if you keep training and pushing to your limit occasionally, you'll keep your muscles—and your metabolism."
Simply put, muscles burn energy, even at rest. Fat stores it.
Bass cites the work of two of my heroes and mentors, Dr. Ancel Keys and Dr. Larry Lamb, who said: "Much of the loss of muscle as we age is preventable—and even reversible."
Bottom line: build lean muscle mass and you'll burn more calories.