I'm pretty sure you cannot retire from a federal component and join the reserves.

But you might be able to get into the guard in some states, I have seen some really weird manpower arrangements in the guard. You might have to forfeit part of your retiremnet pay equal to your guard pay...ie work for free.

As far as flying, you would already need to be winged, I'm not sure if you are or not. If not, you are too old, period...no way you can retire with 20 and still meet the age cutoff for flight training.