You don't have to be 21 (or 18 for that matter) to enter the clubs. Disco and casino - yes 18. My son went to all the clubs because of the music. Of course we went together, but he was a "regular" at the piano bar and had a blast singing along.
I imagine if he had been on his own they might have asked him to leave, but with Mom along it was fine.
Your young adults should be fine.
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