Originally Posted by StuckInYYZ

Do not lie about your age. Priority Pass can suspend your account (and your parents' if linked) if they find out.

That said, the reason you need to be 18 is that most (if not all) their lounges offer alcohol which they cannot allow minors to consume. (and they can't go on your word that you won't consume) You likely would not be allowed in (and quite likely asked to verify your age). Even 18 might not cover it depending on state/country you're trying to access a lounge in.