they get fat if they over-eat simple as that
You'd think, but I suspect it isn't that simple. As one ages their metabolism often slows down. When this happens, people sometimes gain weight. Of course you have to eat too. But what seemed like a reasonable portion when you were thirty years old can be excessive when you are 70 years of age. All my life, I had to work to gain weight. In the last few years, I've found gaining weight is easy while losing it is hard. Although I eat a lot less then I once did, I know that I am also not as active as I was prior to retirement.