I think the big reason people get heavier as they get older is because their lifestyle changes and not so much their metabolism. A slower metabolism will make you pack on 10 or 15 pounds, but all those women who were thin when they were younger but then gain 30 40 50 or more pounds when they get older can't be explained away by a simple slow metabolism. I think the reason older people tend to gain so much weight is because their lifestyle changes... young people are always out of the house, hanging out with friends, moving, doing... older people are more likely to sit at a desk or at home for hours at a time. Plus, being idle for long periods of time makes you want to munch on foods, so you have a double threat to your weight: sedentary lifestyle and excessive calorie consumption.