Well I think many people will agree with me. Really the nightlife depends on the type of people that goes to the cruise. Which also depends on the time of the year that you are sailing. I have only sailed once and my second trip will be on May. But what I am going to explain can be concluded by reading different post in this page and other.
During the month of May trough July, the nightlife probably is better for people between 18-25 years. Why?? Well probably because in this time of the year is when more young people sail. Why?? Well most of the time Senior Trips, Summer Break. So most of High School Students and College Students have the summer of, so there is more probability that they will sail during this period of the year.
The vacation of people 25 and above depends more on them and when they want to sail, the vacation period is not restricted as the period students have.