NIckname confusion

for me I took it as kiddo -- english nickname for a child. Takamura thought he was older, but he found out they were the same age so he jokingly called him kiddo. When he found out they were the same age he could prank him.

I wanna think its japanese version cause it just feels too weird to call same age person kiddo.. especially when they are the more mature one :D

that why it was a joke lol but all ways of thinking about it are cool