I can attest to how strict they are with age. And inflexible, in my opinion.
Last July, we cruised with my sister and her family. My daughter was 9 (so in the 9-11) group and her daughter was 8 and 11 months (really, turned 9 August 12). My sis "snuck" her daughter in with the 9-11 group with my daughter and they called her on it. We explained the whole situation, that we live 5 states away, and that these girls are best friends, and that the younger would be 9 in less than two weeks, etc. It was too bad - ruined Camp Carnival for the girls, they didn't want to split up...