don't stop trying
Hi,
I was once that young keen air cadet. Keep on trying to get the qualifications that you need.Even if you fail the aircrew medical (I did - faulty hearing) keep on looking around for alternative career paths.
Rear crew do a lot of very interesting jobs that you will probably be unaware of. The service has a lot to offer - it is a way of life that I thoroughly enjoyed for 40 odd years (some of them very odd!)
Spend your time in the cadets getting around the air force and do not be afraid to ask any serviceman for advice.
Good luck!