FWA attendee here. I can’t vouch for other con programs, but the 18+ sketchbook swap I attended (which was just an excuse to draw with friends because we were already in the room for the previous panel) opened with panel organizers loudly vetting the room for anyone under the age of 18.
From what I gathered, minors in attendance had special red wristbands to specifically keep them out of these kinds of events. It seemed like the con staff take it seriously because every room with programming inside had these card readers to verify that you were, in fact, an attendee, and old enough for an event when applicable.
I wouldn’t be surprised if some older teens managed to buck the system somehow and stay after dark, but for all intents and purposes, the con staff seemed to take the issue seriously.