There IS a natural increase in weight due to a natural decrease in testosterone and therefore muscle mass and therefore a decrease in caloric usage. If you add a desk job on top of this, the problem is compounded by a serious decrease in activity. If you add a lifestyle of eating like a pig on top of that, you get humans turning into hippos.  I almost effortless maintained my weight at 140 with a job that required walking; although, I was quite accustomed to moderated eating. As I got older, I became 150 while living and eating the same way.  There is also an increase in visceral fat for some reason, but if you read my first sentence, there's a lot of irresponsibility involved in becoming hippo-sized rather than just picking up a little belly fat.