When you book your cruise, you must give the birthdays of everyone in your party. They will most certainly note a birthday! A cake will be given to the birthday boy or girl at dinner one night during the cruise. It's not the most extravagant of cakes, but it's still nice.
You can also purchase a birthday "package." It includes a door magnet
, room decorations
(lots of them) and a cake
that will be waiting for you in your cabin. You'lll see it listed on the DCL site and there will also be an insert within your cruise documents.
Hope that helps!