I figure she is around 14-16 years old. But like the first comentario dicho it officially states she is 16 years old at the bottom of the 4th page in the amazonas, amazon preview.
I tried to figure that out after seeing the movie, and decided that she was probably anywhere from 16 to like, 21 o 22. dado that 16 seems a popular age for disney princesses as well as for marrying girls off way back in the day, I kind of settled on that. Like Ariel, she can pass for a 16-year-old. And then I flipped through the junior novelization at Barnes & Noble and discovered that I was right. ^^