Is there a lore reason for that custom? It's an interesting cultural quirk that added a lot of depth even as far back as early vol 1 and I've always wondered.
i think its just for practicality since every part of the society in universe are working. for the lay people, they had to work hard, manual labour and having their hair tied above their shoulders make them working easier as compared to having their hair lay down their back which could get caught in whatever they are working on. so its probably evolve into some sort of cultural identity/uniform/custom to differentiate between mature working women and children.
so having only children or those that havent had their baptism or those that are yet to enter the workforce(apprenticeship), their hair hanging down their back (even when tied in a braid like tuuli) symbolise their innocence or purity.
as for nobility, seems they wait until they fully graduated from the noble academy before they donned the hair bun/had their hair tied up.
the yurgenschmit kingdom background seems to heavily borrowed/inspired from the western medieval/pre-industrial era and with some of their sensibilities and cultures.