I am currently a YAC working for Princess. I am not sure about the other cruise lines but there is no age limit. We have YACs who range in age from 21 up to 60. With Princess, you will start out as youth staff and then you have to work your way up to being YAC which usually takes 2 full time 6 month contracts at least. The pay starts as staff at $1100/month us dollars. The pay as YAC starts at $1900/Month. I hope this answers your questions.
Best of luck,