The first cruise I went on I was only twenty. I was with my future husband and in-laws. I had no problem being served alcohol. At first my husband (boyfriend at the time) was buying them and then giving them to me but soon enough I was using my own card to buy them. I know it sounds bad but it's not like serving to a teenager. I think you will have no problem getting champagne for your birthday, at least I hope not. Cheers to a happy birthday and great vacation.