Things to know: If you think you will get seasick, get one of those patches. It's better to prevent than cure seasickness. Don't be shy about asking your steward for extra pillows or towels or whatever else you may need. These folks love to help you have a great trip. So introduce yourself to your steward as soon as possible after boarding.
If you have packed more than one bag and one carry on each, take a hard look at the clothes you are packing and reduce the pile by about 50%. DO bring dress up clothes. You won't regret it. DON'T bring beach towels -- the ship will provide these for you. DO have at least one ship's photo taken--on dress up night of course. Sign up for any excursions you want to take from home to eliminate standing in line on board or missing something you really want to do. As for the rest--I always tell friends it's best to find out for yourself. After all, half of the fun of travel is learning stuff for yourself.
Re AGE: Most cruises I've been on lately have the full range of ages and it makes for a very nice and interesting experience. You'll find younger folks hanging out at the bars and disco, in the gym, and on the more rigourous land tours, but don't be suprised to see older folks lining up to go snorkeling or working out on the stairmaster!
..also depends on time of year..whether school breaks are in session or not..you are safe with that.. school is still on.....