When I worked there, you had to be 18. Can't hurt to show interest early
I was hired at 22 as a Specialist, and then became a Mac Genius when I was 24. Genius is a much tougher workday than the engineering position I hold now at 30 outside Fruit (and Genius pays about half). You have to handle people's delicate emotions when they've lost every photo they had because their machine's drive failed, or just as bad-- they spilled something in their machine and it costs $1240 to fix- even though they bought the AppleCare. Sometimes you can truly make the difference in a person's day, but a lot of the time it's an uphill battle to win the customer back. That's the art... Telling people bad news (or neutral news) and keeping them happy enough to not destroy the store's customer service score on their survey. In comparison to traditional IT/tech jobs, you're more psychologist as a genius than computer technician. Especially these days, it was 75% Mac / 25% iPod when I started. By the time I left, our traffic was 90% iPhone.
Follow your dreams though, the six years I worked at Apple, I would say we're the best times I had as a twenty-something. I made incredible friendships with like-minded people, I even met my wife through a mutual friend who was my customer and suggested I call her. I learned what makes good retail managers and lousy ones, and of course, I learned how to take apart any Mac in under 10 minutes. I also learned it's nice to have weekends and nights off when you get a little older, which Apple Retail doesn't offer.