The reason your back stiffens as you age. . .
It becomes harder and harder to get a full glimpse of your ever-expanding and/or sagging ass in the mirror. You see, it is a psychological defense mechanism. The less you can see, the more you can convince yourself it is still the same size and shape it once was thus protecting your delicate aging psyche from the truth.
If you are lucky, by the time you have a true old-lady's bottom -- wrinkled, drooping and held into place by giant panties -- the combination of stiff back, poor eyesight and the onset of dementia will allow you to enter your golden years convinced that your ass is still as firm and round as it was when you were sixteen.
What you can't see, can't hurt you.