Title of movie
The whole premise of the movie is that when Cody enters into the Thirteenth Year of his life, he starts turning into a mermaid, right?
Well, you enter into the 13th year of your life when you turn 12. You end it when you turn 13. Think about it: when you turn 1 year old, you are finishing your first year and entering into year 2. When you turn 2, you are finishing the second year and entering into year 3. And so on.
Therefore the movie as is should be "The Fourteenth Year", as when he turns 13 he begins year 14