I’ve found that players don’t need to know all the rules, they just need to know they are there and available for their reference, and know what their stats and skills do. Just explain what they are rolling for and why as you go, don’t skip over rules arbitrarily to make it easier (i.e. follow the same combat sequence every time, you can leave stuff like advanced initiative out but be consistent in what has to be rolled and what doesn’t), and let them know stuff their character would know (i.e. if the character is taking a shot that’s going to require a really good roll to be successful, let them know it’s going to be difficult and what they will need to roll), and they will soon be quite proficient in the rules. I’ve run games of AD&D 2E, Call of Cthulhu, Star Frontiers, and a science-fiction one I made up with people who were COMPLETELY new to role-playing, and they all became good players (better than many long-time players I’ve known but probably because I am strict with the rules but make my campaigns very open-ended).
I taught myself to play D&D (the red box, I believe) when I was 8, and since I knew nobody who played I had to DM for my friends and relatives. I think a reasonably bright 8 year old should have no problem learning to play.
