In the मांगा Kisa is actually older then Hiro. Kisa ranges from beginning of the मांगा to the end to 12-14 years of age, while Hiro ranges from beginning of मांगा to the end 11-13 years of age.
In the ऐनीमे they are also about 1 साल apart in age with Kisa being older and Hiro being younger.