Carnival's policy for the disco is that no one under 18 can be there after 11pm. So - you are safe for all hours it is opened. You can certainly hang with anyone you wish regardless of their age.
The teen nightlife really depends on what time of year the cruise is. You will have more teens in the summer or during holidays. Carnival is quite strict about the drinking age being 21. They enforce it rigorously. Royal Caribbean allows 18 year olds to drink beer if local law allows it while in port. RCCL also allows 18 year olds to drink beer while at sea.
You can participate in anything you want at the age of 18. About the only thing you can't do is drink alcohol. The older teens on the cruises I have been on seem to have a big time without getting tanked. Depending on the ship, the arcade is a big hangout. And for some reason the upper decks around the main pools are teen magnets. I am not sure why.