As for cruise lines, they all have their pluses and minuses. What might be great for one person might be less than perfect for you.
Carnival has had a reputation as a party cruise. We are taking a Carnival cruise in February to see if this reputation has changed or is still deserved.
Royal Caribbean puts our a very nice cruise product. There ships are very interesting with lots of things to do. I find their food more than acceptable. This might be a good first time cruise if you like lots of activities.
Princess in my favorite line and I have been on them more than any other. I think they have the best mix of great ships, good food, nice atmosphere, and good mix of passengers. I find the Princess ships to be quieter than some others and would not recommend them to someone looking for a "party" cruise.
Celebrity is very nice. They have excellant food and beautiful ships. The only reason I choose Princess over Celebrity is that Princess is usually a couple of hundred dollars per person less. In my opinion, Celebrity has the classiest ships.
You will find people of your age on all of the lines; but you will probably find more on Carnival and RCL.