One advantage to booking early is cabin location and the choice of the cabin catagory you want. If that doesn't matter much, then book when you can afford it, although when first booking, you will just need to put down the deposit on a credit card. The final payment is usually around 60 days prior to the cruise, so if there is a special cabin you want, booking early is the key. Most travel agents will adjust your price if it does go lower after you book, but you can also keep an eye out on the prices.
As for the ages of cruisers, you will find a bit older crowd on the longer cruises, usually over 7 days, so while there probibaly won't be that many cruisers in your age group, there should be some.
You may also want to post on the "bargain finder" here to get some prices and availability, its a good place to start, and travel agents will e-mail you back with what they have on the cruise you list, its very easy and fast, good luck and Congrats on the upcoming wedding/honeymoon!
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