I doubt you could pass a flight physical or even get a waiver if your knees were bad enough for a discharge.

Also the usual age limit for starting flight training is 28-29...you would have to complete the application process, complete OCS/OTS, and actually start flight school before your birthday. Waivers are possible, but tough (you might have to be a real standout).

Right now I think the AF requires UAV pilots to have a flight physical but I kind of doubt they will stick with that forever. Unfortunately UAV pilots are currently pulled from the same pipeline as other pilots so I assume all of the same rules apply. I think this will change eventually, but probably not soon enough for you.

Other services have enlisted folks operating smaller UAV's, might look into that.