Older folks like me have another problem. The butt decides to pack its bags and move down south (otherwise known as sag! )
To coax it back, I've been doing:
stiff-leg and/or Romanian dead-lifts
squats
walking lunges
single-leg leg presses
treadmill walking on a steep incline
going up the stairs 2 or 3 at a time
Another thing I do is lie face-down on a flat bench with my legs hanging over the edge. Then I lift my legs and hold, concentrating on squeezing my glutes hard for a few seconds at the top. When that gets easy, do it holding a dumbell between your ankles. Be careful not to over arch in the back, just go up far enough to get a good glute squeeze.
These exercises are good butt builders or elavators, just really concentrate on using your glutes in each exercise.
Also, if you have access to a rowing machine, this is a very fine way to TOAST those buns.