Logically Rapunzel, Cinderella and Belle are the next oldest, physically (K, maybe not so much Rapunzel, but she's stated to be turning 18) and psychologically they fit the 17-19 window, they are young women, not teens. All the girls are either said to be 16 in the movies, or have the bodies and temperaments of 15-17 at most. I'm going to go with Cinderella because I think she's 19, she just has the feeling to her.

I think her and Tiana are the same age and like Jesikaroo said, Cinderella, Belle, and Rapunzel are referred to and treated like young women in their films. I think all of the others are 16.

I always though Belle is in her early 20s.

Then Rapunzel, Mulan, Belle, Ariel, Aurora

I feel like Belle is 19 or 20.

i believe Belle is older than Cinderella, she looks like 19, while Cinderella looks like 18

She just seems more mature, Belle would be next maturity-wise, and Rapunzel is stated as 18. Then Mulan, she feels 17-18 to me. After that, they are all stated as 16.