A lean body mass will increase metabolism. It is certainly possible to delay the decline in metabolism as you grow older even though the decline is natural. Muscle volume is a determinant of metabolism that will take away fat or calories. There is no way around it that exercise is important.
Weight training at least a couple of times a week will build muscle and strength and you can fill in the gaps between workouts with lighter easier exercises. Even things like taking the dog for walks and avoiding the elevator and taking the stairs can serve over time to help increase metabolism and in turn burn calories.