My Wife and I have been married now for six years... When we got married we opted for a down payment on a house instead of the Honeymoon. Six years(and three children) later and we are definitely ready for a real vacation. We are in our late 20's and are looking for something not quite as rowdy as the lines catering to the party crowd. Our only requirement are sun, sandy ports, and a good time.
Congrats on planning your delayed honeymoon. The only problem with your request is that most of the cruises heavy on 20 somethings have somewhat of a party rep.
If you want ships with plenty of people your age look at Carnival, Royal Caribbean, and NCL's newer ships. If you want to avoid the party thing all together try Cunard or Holland America, mostly senior. Somewhere inbetween would be Princess & Celebrity. I know I'm missing some but these are the mainstrem lines with plenty of choices.
The time of year you sail and your destination will have nearly as big an impact on how many children and partiers there will be. Less on longer sailings (7 days or more) more on short sails.
Paek party season is spring break but will include summer months and any time kiddos are out of school.
Have a great vacation, hope this helps. I'll leave the rest to the other here who have strong preferences on what cruiseline you should opt for.
Congrats!! You definitely deserve a vacation and what a great way to celebrate. We've been on NCL, RCL, HAL, Costa, Carnival, and Princess. To me, in my opinion, Princess does it best. Their ships are immaculate, the service is impeccable, and the food is great. We've only sailed on the Sun class ships and are booked on the Emerald for November. The Emerald is a sister to the Crown, and is larger than the Sun class ships-Dawn, Sea, and Sun, so I'm really excited to try her out. As Neil stated, the longer the cruise, normally the older the crowd. Princess has a good mix of age groups and not as many children as Carnival, Costa or Royal. Some friends of mine cruised on Celebrity and really enjoyed it. Celebrity is a little more expensive though. Princess is competitive with the other major lines. The other thing I like about Princess is the personal choice dining so you are not tied down to early or late seating for dinner. This makes it easier during port days for us. We don't have to rush to get back for early seating or starve waiting for late seating. (I can't go grab a piece of pizza and then go to dinner--just can't eat that much so I wait for dinner). The Caribbean is great for beaches and blue water. Mexico is nice, but my preference is the Caribbean. Best wishes and hope this helps a little!
Regardless of what ship you travel on, there is always the chance of a loud group somewhere, and those groups can be avoided. Penny is right with the mix on Princess. For the late honeymoon, I would recommend the Caribbean Princess which has great balcony specials and a relaxing itinerary.