That's a new one. Are you on a PC using Windows? (Just checking to see if you're emulating Windows).
Yes, I'm on a Dell PC using Win 10 Pro. A little more detail - if I move the cursor outside the window, it unchecks the box. As soon as the cursor moves inside the window, it checks the "under 18" box again, this happens both in windowed and full screen if I move the cursor up out of the game screen. I've tried hitting the <Tab> key, The F1, F2, etc. keys, and the arrow keys with no effect. Good luck with this one!