Although I'm no expert in human biology,(don't play one on TV, either), I think it is pretty well excepted that our metabolism goes down as we age. I think that is why it is so hard to maintain a certain level of conditioning without an increasing level of exercise, whether aerobic or weight training. At 51, it is increasingly difficult to maintain my present weight and fitness, even by eating more wisely, riding 60-80 miles a week, walking with my wife, and doing my job, which is fairly physical itself( I'm a farmer).

This winter, I will probably add swimming back to the exercise mix, two nights a week. I did this several years ago in addition to my other activities and it really helps overall body condition. Low impact, too.