It may have happened 20 years ago, but your just finding out now. Of course it hurts. I think the fact that it was hidden for so many years would bother me too. No one likes to be lied to and you were.
If your wife is willing and offering mc, i would take her up on it.
While its a hurtful thing to learn, it was a long time ago and you state things have been well between the two of you. Take her up on the polygraph too if she is offering.
She seems genuinely eager and willing to help you heal. Those actions speak volumes for your wife. You are a fortunate man.
Its a difficult time now in your marriage, but fortunately for you, with some proper healing, you and wife should be able to move forward together with a stronger bond.
I think it may do well for you to consider the fact that her lying was based in her feeling shamed for the choice she made and not wanting to hurt you.
I'm not excusing her lie, just stating its understandable why she chose to do so. She was truly young when she made this mistake and i imagine as life went on and became more involved, it became more and more difficult to tell you.
I would do my best to forgive and move on.