Your metabolism does slow as you age, this is a minor factor, but the major factor seems to be lack of activity.
Now this may seem wrong at first, till you examine why.
After all exercise doesn’t really burn a lot of calories. If you try to lose weight through cardio exercise, you will only lose about 5lbs, maybe 10lbs if you are really able to push it. An hour of high impact aerobics only burns an average of 300 calories. As you can see a candy bar has about 220 - 280 calories.
So you can pretty much undo a whole aerobics class with one candy bar.
But studies in youth have found active kids weigh less. But why? The key is not that their burning calories, but they are NOT eating.
The three top reasons people eat are: “Watching TV/Movies,” “While on the computer,” or “boredom.”
You see the more active you are the less likely you are to be eating mindlessly. If you’re kid is playing baseball, he’s not really burning off that many calories. But he’s not at home with one hand on the video game and one hand in the Cheetos.
Same goes as we age.
I volunteer in a retirement home and a few of the residents have coughing spells at night which they quiet by eating crackers or bread. This works, I guess it traps the mucus or whatever, but it also mean a lot of extra calories without realizing it.