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