"Assume that there are two students, one who starts training first and the second who starts training a few months later. If the first student stops training and while the second student continues to practice, what happens if the first student returns to training 20 years later? Who is the sempai (senior)?"This is a common situation. After 20 years, the second student might be a 4th or 5th dan. The first student might not be a black belt at all. The time period could vary. It could be 30 years or longer.
The general feeling was that the first student remains the sempai. The sempai is the student who started training first and this would never change.
One of the sensei explained that he based sempai on who attains shodan first. In the absence of a ranking system, the sempai would be the student who learned the kata curriculum first.
However the sempai status is determined, there could easily be a situation where a kohai (junior) has a higher rank and possibly higher title than his sempai. To me, this is fine. The sempai relationship is one of respect, not rank. One will always respect the students who came before him or her. It is a little bit like saying that your uncle will always be your uncle -- even if he hibernated for 20 years.
But the sempai will not abuse his status and will realize that his kohai has learned much more than he in the last 20 or 30 years. He will defer to the kohai and insist that the other students show the proper respect to the kohai.
A typical conversation might go like this:
Kohai: "This is my senior."But the kohai will always be respectful to his sempai and avoid doing anything to embarrass him (such as defeating him blatantly in kumite or teaching him very basic things in front of other students). The sempai, in turn, will support the kohai, particularly if he has become the dojo sensei.
Sempai: "No, no no. I have not trained for many years."
It is extremely important to remember that seniority is not based on rank. I may have a higher rank than many people who are senior to me. I tend to address such people as "sensei", even if they do not head a dojo. There are many practical reasons why people with superior ability might not receive rank as quickly or as numerically high as others. Rank can be a pretty subjective thing. One should never be blinded by it. Many pre-war Karate students, particularly in Okinawa (and Hawaii), had no rank at all.
And your own sensei is always your senior. There are no circumstances under which a student will become senior to his own sensei. Your sensei is and will always be your sensei.
For returning students, a black belt who has been aways for a number of years will tend to wear a white belt until he has gotten back into shape and refamiliarized himself with the dojo curriculum. He will first ask the dojo sensei for permission to return to the dojo and, if invited to return, will do his best to learn what is currently being taught. He will not dwell in the past and practice things that have been discontinued. It is important for returning students to be very polite and sensitive to the attention they might attract among the junior students.
I should add that in my experience, we never address a person as "sempai" or "kohai". These are terms to describe relative postions like higher/lower, younger/older, bigger/smaller. They are not formal titles used in Karate.
Charles C. Goodin