I have noticed that even young men and women who have lost a lot of body fat (even those who were thin to begin with) experience what I refer to as sliding body parts. The ass slides down the back of the thighs into oblivion and then you don't know where it went. The next thing you know you're looking in the mirror and your cheeks slid down your face and fell off.
As you age or if you lose body fat you lose that volume and it is hard to compensate after. Exercise is the best thing to help a disappearing rear end and such. If your cheeks fall off well short of injectable fillers which could make you look like one of those house wives of NY ( not pretty on a man) there are not many options.