Just like punisher 29073, I’ve been thinking about trying BJJ. What things should an 40 something year old guy look for in a BJJ school if they wanted to get started?
I'm 47 - started BJJ when I was 41. I don't know that I'd suggest anything all that different from what I'd tell anyone else.
You'd probably be best avoiding a class that's full of 18-20 year old kids - or at least find one that has guys late 20s and up involved, as well. You can easily roll with the youngsters - you're likely to find that, depending on your conditioning, you'll have to play a different (less hard charging) game than they will, but that's good for you. The issue is that many times, there'll be a couple of guys in a squad like that who don't yet understand that their goal shouldn't be to break everyone they roll with. They think everything is a tournament. So, having some guys in the class who aren't so "win at all costs" about it is a useful thing to avoid injury.
Your best bet might be to call or stop in to a couple of schools in your area and talk to the professor. Find one that you have a rapport with, and start there. Ask to observe a class, if you want, too. It will take a little while to learn how to gauge the quality of the school, but if you have any friends that roll already, they might be able to help, there. The spirit you're looking for is a place where guys are having fun, and it looks like competitive "play", not all out war - and a place where the students are helping one another learn cooperatively.
Tap early, tap often, and without reservation. Leave your ego at the door - it's about learning, not winning, especially in the beginning. Expect to feel some panic at times, and to feel uncomfortable for a while. Those things will pass, but it's part of the deal until you learn to relax under that stress. You're going to spend your first six months or so wondering just why in the hell everyone is tapping you, and you can't seem to remember anything you've been taught, much less actually tap someone out. It's all good, and it will improve!