Sadao Yoshioka Sensei had the following to say about the Sempai/Kohai relationship:
". . .going back to student position, if two students start Aikido the same day, the older person will be sempai, or senior. When two students take the same kyu test at the same time, the older person will be sempai, or senior. Regardless of when you start, if someone who is your junior puts in more hours and takes the next promotion test before you, that person, who was your junior, will be your sempai from then on. So attending class regularly, and taking tests when you're qualified is very important in the long run, because of your status as sempai or kohai. With two persons taking the same test at the same time, age will make a difference, and the older person will be senior, sempai. When you receive your black belt, the date of your test for promotion will be considered your date of sempai/kohai."