Japan is a country of hierarchical relationships.
You show respect to people older than you because they have gathered more experience in life.
In the same way, people who have entered your company or school before you regardless of age is “senpai” (senior), and even those older than you but entered after you are “kohai” (junior).
When your senpai is younger than you, it can feel strange.
Here are some points about the mysterious “senpai and kohai” relationship.