It happens. A lot of local shows are 18+, and once I turned 18 it seemed they all became 21+. I recently missed... damn, what was it..? Magus Beast, Icarus Witch, and... I want to say Attacker was the headliner, I don't remember, but it was a sweet show.
It's much less common for bigger shows like Bal-Sagoth to be 18+, at least around here.