I'm 21 at the moment, almost 22, and my metabolism hasn't slowed down at any appreciable rate. However, I am also eating pretty well, and I get more exercise than I used to because I'm forced to walk around my college campus which, while not huge, isn't terribly small either.
Last year I moved into the dorms (I'd been living at home while I took classes at a Community College) and the quality of my food decreased pretty dramatically. This year, I'm living in an apartment and cook my own food, but I probably get about equal levels of exercise and I'm doing pretty well. I do eat some fairly fattening foods regularly (we have a grill we call the baconator, and Costco sells 4 meals worth of bacon for myself and my roomate for $10. Do the math) but in general I have a healthier and tastier diet than last year in the dorms. So I'm not sure which factors are the most important here, but I'm not experiencing any significant problems.