Wait wait.
Isn't this ageism?
I mean, you are not allowed to deny service to someone just because they happen to be a certain age unless they provoked you otherwise.
Either your establishment is 18 and over only, or you let everyone in. There are no exceptions. This is bullstuff.
Call 14th amendment on their asses. This stuff isn't legal. If you've done nothing to deserve this treatment, then as a loving paying customer you have every right to loving sit in there.