Welcome to cruise-chat.
I've been on RCI cruises that didn't even have the curfew, but our last two cruises with them did have it so perhaps they standardized it.
Security and the bar staffs do enforce it. Realistically, I doubt they card every teenager they see and someone who is 15 but could pass for 20 probably skates by. But it is a rule and I never encourage people to break the rules.