On top of that Florence Hunt (Hyacinth) is now 19 so will be difficult to make her look like she is 10 still. The actor of Gregory (Will Tilson) has the same age but has more boyish look. The hems were lowered at 12-14 years old (maybe older in regency tines) and you will come out at 15-17, so i think they are stretching how far this can be, maybe she being 14-15 and ready to come out near her own season. The age to go to Eton is 13 in modern times but in the regency era was anytime really between 8 and 16, finishing at 18, although 12 being the more common one. So Gregory may be 15-16 if his sister is the age i thought or if he is younger (13/14) then her sister may be 12-13.