18
After Rapunzel announces that her birthday was today, who responds with, "No no no, can't be. I distinctly remember, your birthday was last year"?
Answer: Mother Gothel
Rapunzel tells this to Mother Gothel while still stuck inside her tower, before her adventure with Flynn Rider. She starts the conversation with, "So mother, earlier I was saying tomorrow is a really big day, and you didn't really respond, so I'm just gonna tell you: IT'S MY BIRTHDAY! Tada!" When Mother Gothel tells Rapunzel that her birthday was last year, Rapunzel tells her, "That's the funny thing about birthdays, they're kind of an annual thing."