I also forgot to mention that I think it would be a good idea in regards to the dance that their be an all ages portion prior to the 18+/21+ dance. Again similar to how AX did it, with the wristbands in mind, they had an all ages portion, then by a certain time they asked all attendees to leave momentarily (at this point they switched out DJ's, adjusted the lighting, and made other preparations. They then asked everyone wishing to (re)enter to have their badges and identifying wristbands to have them visible.
If I remember correctly it was something like 6pm-10pm was all ages, then from then until closing it was 18+ (obviously these times would be adjusted to the Fanime schedule, but this is just to give you a general idea)
last year, the ax dance has an occupancy limit of <400 (four hundred) with a second 21+ room of ~300. I don't remember them closing to kick anyone out at 10, they just didn't let anyone under 18 in after 10. the fanime dance on sat. had >4000 (four thousand), it takes 30 minutes to clear the room and asking everyone to exit and WAIT IN LINE again is a bad use of resources (and it won't make a lot of people happy). Besides, the dance is not a dating service and please state the benefit for having age restrictions on the dance after a certain time and how wristbands could be financially feasible and justifiable. as someone that works with tyvek wristbands every weekend, there are ways to easily take them off and besides, if you put it on too tight, they sue you. as for the plastic ones at a hospital (since i used to work at one) you only need a pair of needle nose pliers or a quality hole puncher.