You may have already answered your own question, as in, if you're thinking about it just do it. Last thing in the world you want crossing your thoughts on course at speed is 'is my back protector too old?'.
I've never seen a replacement metric for these either. Would guess common sense comes into play if it’s physically worn (straps, buckles) or is any part of it made of any form of foam, soft or hardened. If it contains foam I’d replace it at least every 5 years like a helmet because that stuff breaks down after being exposed to elements, sweat...etc At the same time, no racing environments have a requirement, suits can be older than 5 years and be just fine (but usually don’t contain foam either).
So, in summary, dunno.