"Safe" isn't a calibrated measurement. A 20 year old tire will have less friction than a newer one and is more likely to rupture under pressure.
With that said, the probability of even an old tire spontaneously self-destructing while JRA is quite slim.
I'd say that it's probably still usable, but not as safe as a new tire would be. For what they cost, and for the trouble they can save you - get a new one.
If you're set on using it - try over inflating it a while, see if it holds up. If it does, lower the pressure and ride.