I believe when they first met, and the first thong was snapped, she was under 20. I remember reading something about how the White House was at pains to try to emphasize an older age for her. In any case, the conduct was reprehensible. I remember that also. The look on the Clitonista's collective face was as if they had dodged a bullet and were looking to see if anyone noticed. Of course, no Republican made an issue out of it.
I have since found that Monica was born in July 1973. So it may be the issue was over whether she had reached 21 or not.