I was actually pointing out she's pretty old compared to other Disney princesses, I think you may be mixing me and someone else's comment. I'm just pointing out her age from what we know.

As far as the birthday bit, it was discussed in a making of and in interviews, specifically there was one with Linda Woolverton who got brought in for a complete rewrite after it was initially presented in the late 80s. She also wrote for The Lion King and Mulan, and the screenplay for Maleficent. She specifically was responsible for the love of books by Belle, actually as part of a direct response to Ariel being... Well, kind of dull as a character.

Regardless of 17 or early 20s, provincial life would have had her - from a societal perspective - looking for suitors beginning around 17. She was considered an adult, and that was the reason for the birthday scene/reference (which got tossed). Given she had been saying no to everyone and there were whispers, that also leans toward her being juuust past that acceptable age, which would have been over 19.

So as I said, anywhere from 17 to early 20s but not a hard definition in the movie.