I currently have 3 Sophie cosplays in progress and I based it in 3 different specific time periods which are 1864, 1894, and 1905 and I interpreted that the dress Sophie wears was given to her mother before she was born in 1864 then given to her in 1894 (her mother being in her mid 40s living in the royal city) since it was common practice to pass along the clothes to the eldest child during the time and Sophie altered it to fit her using her sewing skills and when she met and fell in love with Howl, her dress was updated by Howl into the latest fashions of 1905 since I think it took a long time between working in the hat shop, being cursed by the wicked witch of the waste, working for Howl in his castle, and falling in love with him by the end of the film. By the end of the film, Sophie would be wearing the latest fashions since Howl could magic up her wardrobe for her.
