How old were you when you decided you wanted to attend a service academy (or specifically the Air Force Academy)?
I had a desire to come starting around 8th grade, but I really decided it's what I wanted during my freshman year.
Whenever you decide, as long as you have been working hard you have a shot!
Do ABSOLUTELY everything you can. Admission is getting more and more difficult, with next year's class admitting even less than we did.