Actually as we get older, our metabolism rate slows down, and we end up using less calories wach day than we did when we were younger. Post menopausal women also go through hormonal changes that make life less comfortable, at least for a certain period.

Unfortunately, there's no "magic pill" that can be taken no matter how old you are. The only thing that really works is consuming less calories than you spend meaning you have to maintain a pretty steady level of activity, and watch your diet.