Lovely photo, and they look quite an affluent family
I think it is the complete family, the Girls being the most difficult to age. My view is that the extreme right one tends to look older but because of her hair style and countenance. They all have the same nose profile, but I offer a different opinion of the eldest - Mother 50, Father about the same, youngest son 12,next son 16, Soldier 21, daughter to Soldiers left [ at photo],being the eldest aged 24, daughter extreme right 22, and final daughter 19. I think it could be a family photo to mark the safe return of the Soldier son prior to his discharge. However Jim1 expertise on the uniform makes the birth / photo dates important. As a relative with such a good photo, I would be seeking WW1 records to try and confirm entry etc.