The clear inference is that a slowing of metabolism is inevitable, like night follows day. That's simply not true.
Women often gain proportionately more.
The reason, however, is not an aging metabolism. One Criterion Energy requirement of muscle remains constant Exercise Physiology , the excellent textbook by Drs. There is probably no biologic reason for men and women to get fatter as they grow older. Increases in bodyfat, they explain, are more a function of activity than age. Inactivity results in loss of muscle.