Twenty year old rigging is old. I, personally, would not cross an ocean with it--no matter how good it looked. Here is why.
Stainless steel rigging does not typically fail because of visible corrosion, assuming that good quality 316 or 316L wire was used. The failure mode is much more insidious because you can not see it happening. The rigger who looks, no matter how closely, at old rigging and pronounces it "good for another 15 years" does NOT know what he is talking about.
In the language of the metallurgist to "work" a metal is to bend or stretch it. When you "work" the kinds of stainless steel that are used in rigging the metal hardens. As it hardens it also gets brittle and weaker. Think about bending a stiff piece of wire back and forth. It bends, it bends, it bends, it breaks. Nothing visible happens. until the strands of the wire start to break.
Now, a well tuned sailboat rig (i.e., nice tight wires) doesn't "work" its wires a much, but it does work them. "Work" happens every time they stretch a little tiny bit in strong winds, or in the shock load imposed by hitting a big wave.
In addition to work hardening, there is also crevice corrosion to worry about. It also happens even to the best stainless when chloride is around. Without a detailed dye test (preferably magnaflux) it can be very hard to see, if it is visible at all. This happens faster in saltier water (I.e., The Med), and warmer temperatures (i.e., everywhere we like to sail!)
The problem with any recommendation on this subject is that it is an odds game. The chances of 5 year old rigging that has been in a well tuned rig failing due to work-hardening or crevice corrosion are near zero. The odds of a 30 year old rig being significantly weaker than designed is very high.
You have to draw the line somewhere. A conservative number is 15 years, based on the opinions of lots of people whose opinion I trust. Is it conservative? For most boats, under most condidtions--yes. But in the middle of the ocean, I am a very conservative guy.
When I worked for a charter company with boats that were sailed in strong winds a LOT, we changed small boat rigging every five years, and big boat rigging every seven.