Who decides these ages? They certainly weren't mentioned in the movie.
Allot the extraneous material, sometimes written (or started) before they even complete the movie, descriptions sometimes taken from first drafts, or casting calls that don't reflect the final film, yet still work their way into a book), should hopefully be taken only loosely.
Sure the editor of a book, or comic series will tell you it's all absolute "canon" the "story-group" approved, they want to sell you a book, but nobody (fans and especially creators) hopefully should not have to slavishly adhere to that, especially when it flies in the face of what they are building LOL!