Well it's bound to degrade, as it is lubricating a high speed hydraulic pump and steering rack. It is likely full of wear particles, hence the discolouration.

and not forgetting oxidation. Most PAS systems are open to the air at the reservoir.

there'll also be some breakdown of the polymer chains as a result of mechanical and thermal stress.

I once asked this question of a colleague responsible for PAS systems. He replied that he would change the PAS fluid at 10 years or 100k miles. It should last the "life" of the car, but bear in mind the design "life" is 12 years or 120k (or thereabouts).
I went on to ignore his advice (actually just simply forgot) with my Rover. It never even needed top up in 167k and 15 years and was running fine.

It's hardly a catastrophe if the power steering pump does lose pumping pressure. I notice the steering become progressively heavier as the fluid warms up and loses its viscosity.