Elcapitan08,
As others have already stated, it depends which service you wish to join, and which branch of their reserves.
Some of the most frequent and rewarding flying (albeit pretty basic if you drive airliners) to be had is with the Air Cadets Volunteer Gliding Squadrons (VGS) which are based at various locations around the UK. If you are interested in this, contact HQ Air Cadets at RAFC Cranwell - 01400 261201 is the main swichboard.
You can join a VGS as a Civilian Instructor (turn up, fly, go home), or if you are willing/able to commit more fully to a management role on the unit you can apply to be commissioned into the RAF Volunteer Reserve (Training).
Good fun, good people, good flying, and there is nothing better the smile on the face of a 16 year old cadet who has just flown their first solo.
Other people will no doubt be able to advise you better on more pointy and and/or menacing options.
Cheers
Ash679