We were 30 & 32 when we first cruised and haven't looked back!
We have been on the NCL Epic round the med which is a varied mix of people, but has a pretty lively night scene (Bliss nightclub, Spice H2O, Duelling Pianos, Casino) - although it's a port intensive cruise with some early starts it's possible to balance the late nights and early mornings with some planning! You could continue drinking until 2-4am. They also show all the sport in the bars and on the TVs in your room.
Thomson Majesty (older and smaller than the Discovery) was a brilliant trip. There weren't many younger people at first glance but actually the atmosphere was brilliant at night. People loved a laugh and the staff went out and enjoyed themselves. It wasn't cringe or forced wither, just general having a laugh without rowdiness. The downside for Mr C was the lack of sport shown, they didn't show football so when we go again we have to apparently time it around the football season!
I have to say P&O was our least favourite for night life - it often stopped serving earlier and the formal dining was far too stuffy for us. Mr C didn't want to wear a shirt and trousers for dinner! I also felt that the cliental were more reserved and less welcoming. I'm sure all the ships are different though, and that was just one week, bit given the formal dining we wouldn't be doing it again.
Have fun!