The Casinos and Disco are both 18, not 21.
And while they can not drink, there are lots of adults who cruise for which drinking isn't something they do.
There will be one or two "Singles" meetings at the Piano bar (just watch the Capers the first couple of nights) for different age groups.
This will help them meet others. Our son is 20 and has been on several cruises since he turned 18. He never had a problem finding some folks to hang out with.