I know that when I write, I'm writing for people who can handle high-school math, read at the Grade 12 level, and appreciate subtle humor as opposed to the toilet-bowl kind. I guess that makes the lower cutoff about 17-18 years old.

I agree. When I hear a player talking about how he "needs" to know about all the other details, I hear a player who is being adversarial to the GM and simply not trusting him to "get it right." Players like that make baby seals die and cause cancer with prolonged exposure. I don't write books for them.