Muscle burns more calories than fat. As people age it is common for them to lose muscle tone and increase fat. Even eating the same number of calories will result in weight gain then. If you haven't taken care of yourself, it may not be too late. Improving diet and exercise helps people build muscle even as they age. Not only will this help with weight gain it will help you remain independent. It's not too late to change your habits yet it's better to do it as early as possible.