Our metabolisms steadily slow down following our teens. That's why teens can get away with eating burgers, while middle-agers earn big guts (men) and saddle bags (women) from the same diet. Further, the teens are enlarging their fat cells from poor nutrition, a fact to which they are oblivious until around age thirty.
On the other side of the ledger is exercise and lots of it. Make it fun. It's the only permanent way and best way to stay in fighting trim. Hear that Freepers?