I think its 18, and the last night is the usual night for the talent show. On the elation, it was the second to last night. The best talent show by far was on my cruise on the Conquest. It was absolutely great -- and hilarious at the same time! They had 3 couples (one just married, one married 25ish years, and one married about 60 years). They asked what man would do ANYTHING for his wife and picked these three. Well, the guys came out in speedos and danced around their wifes while they were crawling under the chair in embarrassment. It was sooooooo funny - especially the 80 year old out there dancing in speedos. He was a super trooper! The other talents were singing and absolutely beautiful voices. I actually bought the tape! This past cruise on the Elation talent show was one I could have passed up -- They do have a talent show usually for the teens, although I would think if a 16 year old were talented enough and mature enough, they should put him/her on stage!
They have both adult and youth talent shows. IIRK the age limit is 18 for the adult ones but the CD may make an exception is the person is very talented. I seem to remember a coule of rather young people performing a couple times. One on the Piano and one singing.
I have sung in 10 of our last 12 cruises with Carnival. The age limit on all those shows was 16. I have seen them "bend" that rule. On the Conquest, a young man only 13 played the piano. He was very good. A young lady performed an "expressive dance" routine that was still on high school on the Spirit a couple of weeks ago. They did not ever ask her for her age. She was excellent.
I am sure it is at the decretion of the Social Host and it depends on how many "adults" show up and how talented the younger people truly are.