Modern living does something nasty to our appearance: it flattens our butt. Aging does the same thing. Remember erosion in your geology class? Well, the same thing is happening to those firm glutes dealt to you in youth.
We all know that one of the biggest signs of the "grandpa look" is a flat butt. That's not going to score you any points with your honey. Put on some muscle, especially in your rear, and you'll immediately take a few decades off the appearance of your body.
The good news is that you don't have to say bye bye to your butt. No, indeed, you can actually grow your rear as much as you want to. What's the secret? All you have to do is the oldest exercise in the book: squats. Generally speaking, guys will do a weight lifting workout and you'll never hear them complaining about how sore their butt muscles are. Well, that's a sure sign they didn't do their squats.
This works - trust me. I'm a Skinny Bastard and my wife noticed and made comments about you-know-what. That happened when I was 49 years old, so you can do it too.
Here's another secret: don't just do regular squats, but concentrate on wide stance squats. Italian researchers recently looked examined muscle activation in the legs and rear and found something interesting: the width of the squat affected only the glutes (butt muscles).  By width of the squat, I mean how far apart you put your feet. A normal squat is done with the feet fairly close together, but these researchers found that the putting the feet farther apart actually hammered the gluteal muscles much harder.
It's interesting, but I had discovered this principle on my own. I have one knee that bothers me and I found that wide stance squats are a little easier on my knee. The next day I could really feel it in the ol' hind quarters if you know what I mean.
"But isn't squatting hard on your knees and back?", I hear you ask. First of all, if you have a medical condition, talk to a good doc of course. However, for the great majority of you, squats are an excellent health-promoting, butt-building exercise and, if done correctly, will not hurt your knees or back. Of course, if you don't know how to do a squat correctly, get a good trainer.
Here are three tips that will help as well:
1) Do not put your knee over the end of your foot when squatting. This can put unneeded stress on your knees.
2) You do not have to go past 90 degrees to hit your thighs and glutes. In fact, I never go to 90 degrees (as if I was sitting in a chair).
3) You don't have to use a lot of weight. If you feel more comfortable with less weight, then just increase the number of reps that you do. You'll still feel it the next day.
It doesn't matter so much how much weight you use - what matters is that you do it. And don't let fear of falling or tipping stop you: that's what Smith Machines are for.
1) J Strength Cond Res 23(1): 246-250, "The Effect of Stance Width on the Electromyographical Activity of Eight Superficial Thigh Muscles During Back Squat With Different Bar Loads"