Generally speaking, it means that all shows are 21+. Occassionally, a venue will have a show where they are legally allowed to admit those under 21. Without getting into the details of how and why that works, when they do, they will list for that given show something along the lines of "All Ages" or "18+". Sounds like it's the venue's way of saying that they are mostly a 21+ venue but letting the kiddies know that they should always keep their eyes out on the venue for those odd occassions where they are allowed to bend the rules a bit.
Maybe for that night, they are in the process, or considering, making it an all ages show but nothing to date has cleared yet. Don't know. But as Eddie said, wait for Snail to weigh in because he's the ultimate authority here with anything Velvet Lounge related. He'll set you straight.