The "RAF" cadets only exist as part of a CCF (Combined Cadet Force). These are attached to schools. You can't join a CCF unit unless you are a pupil at the school.
The "other" branch is the ATC, "air training corps". Unfortunately the maximum age for new joinees is 16 years and 9 months.
You might like to try the territorial army, or maybe volunteer as a civilian instructor with the cadets, tho I'm not sure what the age limit is.
Matt