I don't know, maybe it's the crown that looks similar to her father's, but she just always seemed more like the sister who would be the oldest.

I think the oldest being Aquata makes sense because her name has to do with water and her color is also blue...it's a silly reason but it works for me.

I think the prequel makes more sense in terms of crowns. Attiha's crown is similar to Triton's and Alana has a tiara. It seemed silly to me in the original movie and tv series that these crowns meant nothing and were just a hair accessory.

I have to agree with KataraLover & PrincessAyeka12. It makes more sense to me to have Attina as the oldest because of her crown. She's like crown princess instead of regular princess.

No good reasons here. I just like this. *shrugs*

The order in the first movie never really felt like an order of Triton's daughters to me, just a random order that the girls announced themselves in. Leaving the star of the show and the youngest to be announced last. May just be me but that's what my impression was. (That and I never gave it much thought as the order of the birth.)

I Have to Agree with the Prequel

The prequel did no research to stick to canon and doesn’t follow continuity with the original movie like the tv series. Besides, the original comics, books, and pretty much ever other media, Aquata was said to be the oldest. Which given that she was the first in the song and has the same color pallet as King Triton, makes perfect sense.