In the Japanese martial arts the understanding of senior and junior (sempai & kohai) is an important dojo teaching to not only promote harmony in the dojo, but also to ensure that each training session is as smooth as possible.
As western students we sometimes approach this relationship from a point of misunderstanding as we are used to being told what to do, compared to the eastern model of watching what to do.
Different dojo definition may exist for this relationship but for the most part they are or can be a combination of seniority- when did they start training in the dojo, and/or what rank do they hold in the dojo.
Equating rank as responsibility.
Consider that everybody who started before you is a sempai and everybody who starts after you is a kohai- as you are the sempai (senior) to them.
Senior/junior does not always equate age relationship, but it could be a factor in the definition.
Again, look to what is being *done* vs. *said* in the dojo.
As martial arts friends we come together in the dojo each week for martial arts instruction, and this time is the most valuable resource in the world.
Every moment of time we want to spend training, receiving instruction, movement, feeling, and transmission from the teacher.
Sempai are responsible for keeping things running in the dojo.
Before class starts are the mats set up?
Are the training weapons out and ready?
Is everything in order so the teacher can walk in and start instruction?
Sempai also mirror how to act in the dojo, based on the rules and tradition of the dojo. Show by leading silently, so the junior (kohai) students can learn how to act, so when they themselves become sempai, they can lead.