Depends on the character's personality and life events.
One of my secondary characters, Fuyuko, has been keeping her hair in a rough short cut because she was orphaned at the age of 8. Long hair is not something she wanted to take care of, and as a street rat, she preferred not being easily spotted as a girl.
But she got adopted at the age of 14, and has started letting her hair grow out. She's still going to put her hair into practical, tight styles if she knows there's a risk of a fight, but she does want to look pretty too. It's come up in story once recently; she asked two other teen girls to help her with her hair, since she wasn't used to it being this long.
Most of the other characters aren't going to have terribly notable hair style changes. Those who like to change their hair a lot are doing so anyway, and others might already be stable. Like there are some teen guys in the story who are basically knights in training, their hair is kept a bit long to be part of the padding underneath their helmets. But it's also kept neat in a style that is easy to tuck up underneath helmets, and not too long because they need to be able to do so quickly.