Responses

Cuz Minato wants to be on par with Mikoto. Like Hayato said, Minato wants to be like Mikoto. I think he knows that fact and embraced it that he's younger than him but he just doesn't want the gap to be of some sort of a boundary between them. He admires him a lot he wants to catch up on to him and be of the same standing.
So they are the same age except one of them is one year older... like what? Σ(っ°Д °;)っ
http://www.mangago.zone/read-manga/yuzuki_san_chi_no_yon_kyoudai/mdx/d_v-1-chapter-1/14/