Sakuya says Kuuya "age-regressed" due to the events she just experienced being too painful to confront, but I wonder if it was actually inflicted on her as punishment for breaking the pact. At least I think the latter explanation would be more interesting, as I don't find random onset retardation to be very compelling. I could kind of understand it with Casca in Berserk, but what she went through was multitudes worse.