It depends on when the cruise was. A few years ago they were letting kids who had just turned 18, but were still in high school to go to the teen clubs, too. Because of an incident in the clubs involving an older teen they discontinued this practice.
The policy of allowing 18s who were in high school or had just graduated ended in December 2011, so any cruise before that could have had an 18 in the teen programming.
Palo is totally strict--you cannot make a reservation for a guest under 18. THey don't have to "card," it just can't happen. The only way I can see it occurring is if the reservation was made in an adult name and then a teen went in place of the adult--if there was a reservation for grandma, grandpa, and the parents but grandma decide she didn't want to do a "fancy dinner", they aren't likely to ask if 4 people show up.
We did DD's 18th birthday in Palo--getting that booked in advance took some intervention from the supervisor (the computer keeps your age at boarding as your age for the entire cruise.)