Maximus De Minimis

Been wrestling with my jeans lately, which keep falling down. Finally figured out what should have been more obvious.

I have Disappearing Butt Syndrome! My gluteus maximus is largely (?) atrophying. I've been excessively sedentary lately, I guess, but also, I'm getting old, I guess. I do get out for walks and a bit of cycling, but I think I need an exercise routine.

Any advice? Can someone recommend a workout routine to put back some muscle and ideally restore my butt to its former size, but at least stop the disappearing act?

Thanks!