If Power steering pumps die, at what age/mileage does it usually occur? What does it sound like when they do? I'm getting a humming noise that goes up in pitch with rpm's. It sounds like the groaning p/s pump sound I've heard on other cars.

It can happen at any time although the Subaru pumps do not generally fail prematurely. The only reported PS failures here have been from people who were doing donuts in the snow or dirt with the wheel at full lock, that will kill the pump quickly.
If you suspect your pump of making noise, you can remove the belt and run the engine, and see if the noise stops. If it does it is either the PS pump, alternator, or idler pulley. With the belt off you can spin them all and see if the noise shows up.
Also with everything together and the engine running, you can use a long thin screwdriver or rod as a stethoscope to find the source of the noise. Just be careful around the moving parts if you do this.