I don't play blue archive but I guess number-wise it's the middle point between a child and a teen.
i.e. at 12 you're still a 'child'(in/about to graduate middle school) and at 13(start of junior highschool) you're now a teen, so 12 yr 6mo is 12.5, right in-between.
Can people stop confusing national age of consent and age of consent, in the US the national age of consent is 0 but it's no lower than 16 in any state (most states it's 16), in Japan national age of consent was 13 but it was 18 or 19 in all prefectures.