You've got to remember that Timmy is the odd one out. Nobody else keeps their fairies as long as he does. That was also in one of his specials: He had them a year, and that was considered outrageously long.
I think the system just isn't designed for people who can keep their fairies. Why else wouldn't someone wish their fairies invisible, and why can't they just wish that no one would notice anything their fairies ever do? Boom—fairies forever.
Still we learned early on that you lose your fairies when you become an adult, so if Timmy still has them at 20, he's a 20-year-old kid. As long as he acts like a kid and keeps his fairies hidden, he can keep them. So Timmy is screwed. (Or he would be, but he doesn't age.)
edited 21st Aug '10 7:23:44 AM by BigT