Nintendo has revealed in an interview with Japanese site Nintendo Dream that they had originally considered having adult Pauline accompany Donkey Kong in his adventures in Donkey Kong Bananza. However, when they put her on Donkey Kong’s back they realised that a smaller figure would be more suitable and would work better so they decided to use a younger version of Pauline.
Motokura: Pauline was one of the characters we originally wanted to appear in this game, but we could not find a way for her to fit well into the gameplay. Once we made a prototype for the Bananza transformations, (Naoto) Kubo made a song from that, and then once we connected the song and the transformations, the inclusion of Pauline was decided.
Why was her age set as 13?
Takahashi: In order to allow them to travel and adventure together, we put her on DK’s back. At first we tried her as an adult but a girl felt better in terms of size, and we thought that this would be the first time for many to play as DK, so as a new character this seemed to fit better.
Thanks to Edward B for sending in the news tip!

Pauline should be with Mario. They could do a game with Yoshi like this with young Mario then this Donkey Kong and Yoshi crossover.
Pauline should be with who she wants. This is 2025 not 1725.
Boomer troll please go away I swear you old people so annoying with your woke nonsense.
I figured this a while back. Could have also just shrunk her down and played more with the parallels to King Kong.
I figured they originally planned this during development of the game. A person can be any size they wanted them to be.
“How about the in universe reason Pauline is young?”
Nintendo: Shut up.