re: A/C trouble in not very old car

Bend a small paperclip or thin wire into a u shape, disconnect the low pressure switch and put each end of the paperclip into the connector. Crank your car, turn on the A/C and see if the compressor kicks on. If it does, the pressure switch is bad OR you are low on freon...most likely low on freon. If it doesn't, it's likely the clutch relay in the under hood fuse box or possibly the compressor clutch is shot.

Edit: the clutch relays are known to go out. Here a diagram...if there's an identical relay in there, just swap it out and see if your A/C kicks on.