Some time ago the webpage suggested that kids about 13 could probably enjoy it (and that you'd be regarded as totally awesome to them if you did!). I think the only thing that a parent might take issue with is the degree to which sexuality is explored, but depending on the relative maturity of the younger guest, it might be just fine for them to enjoy. Still, if it were a film, it would probably wind up somewhere between PG-13 and 18A.
I think high school age might be a bit more appropriate, but I don't think that the 18-year old mark is necessary. I say that mainly because the play itself has a story that tends to get forgotten in the promotional materials. The play offers something to everyone.