At least in the Comedy Club and Quest
Not sure when this started but on our cruise last week on Odyssey they were carding kids at the doors and security was going around removing kids who had made it in prior to the event starting. It is unlike before where they could be there with their parent, now it does not matter, if they are not 18 they are forced to leave regardless of whether their parents are standing beside them or not. I heard the same thing from a friend with a 17 YO on Icon last week.
I am suspect that there started to be too many entitled parents who decided that their 12 yo was mature enough to go so they just made it black or white with no option for parental consent.