Pushing 40? No way, I say you've frozen time so you'll stay early 30s for the next 20 years

Remember Dara Torres just won Olympic medals (and beat a bunch of 20 somethings) in Beijing in her 40s.
There are a bunch of squats exercises in the library - search on "squats" and you should get a ton. You might also try Bulgarian Split Squats where you put one leg behind you on a bench, hold a dumbbell in each hand, and then do a one leg deep squat. Anything going deep will hit your butt pretty good. Mix things up so you're doing some walking lunges, squats, squats with a fitball, etc. and throw in some lateral moves so you're not just training in a vertical plane.
We're all looking forward to progress pics of your backside!