I don't know who you are talking to but my travel agent also said they can be taken to the Purser's office any day of the trip for credit to your sign and sail account. The Carnival Rep I talked to also said the same thing. He said the only reason the 15 day stipulation is given is because it makes for easier bookkeeping. He said many people bring them on the ship and that my children and mother can even wait until Christmas morning or later on the cruise to have them applied to their accounts.
The reason I am slightly disappointed by this is because I had no intention of opening individual accounts for my teens. My mother on the other hand will be thrilled, I am sure!