Between age 20 and 50, most Americans percent body fat doubles.
Slower metabolism has a lot to do with it.
Training with weights becomes more important with increased age. It is well documented that muscle tissue is more metabolically active than fat tissue. This means that as you sit there reading this article, you would be burning more calories with nice tone muscles than you would if your percent body fat was high. As you age, your activity level tends to decrease. With decreased activity, muscle tissue is lost.