It didn't come from Gates...
http://www.snopes.com/language/document/liferule.htm
This list is the work of Charles J. Sykes, author of the book Dumbing Down Our Kids: Why American Children Feel Good About Themselves But Can't Read, Write, Or Add. (The list has appeared in newspapers, although not necessarily in this book.) Many versions of this list omit the last three rules:
Rule No. 12: Smoking does not make you look cool. It makes you look moronic. Next time you're out cruising, watch an 11-year-old with a butt in his mouth. That's what you look like to anyone over 20. Ditto for "expressing yourself" with purple hair and/or pierced body parts.
Rule No. 13: You are not immortal. (See Rule No. 12.) If you are under the impression that living fast, dying young and leaving a beautiful corpse is romantic, you obviously haven't seen one of your peers at room temperature lately.
Rule No. 14: Enjoy this while you can. Sure parents are a pain, school's a bother, and life is depressing. But someday you'll realize how wonderful it was to be a kid. Maybe you should start now. You're welcome.
Advice columnist Ann Landers has printed the first ten items (uncredited) several times, and the list has been used by radio commentator Paul Harvey. The prize for misattribution, however, has to go to The Atlanta Journal and Constitution, which printed the list twice in three weeks in mid-2000, the first time crediting it to "Duluth state Rep. Brooks Coleman of Duluth," and the second time to Bill Gates.
I'm pretty sure it was written by Ted Nugent.
Rule No. 15: These are the Good Old Days.