“OK, I am 50 years old and the instructor is only 25, only a half of my age. Can he be my Sensei?”
To answer this, we have to adjust the time-table to karate time. Suppose he started his karate 10 years ago and you only 5 years ago. He is your Senpai in karate. If he is teaching a class regularly in your dojo then he is your Sensei. In a dojo, the age difference does not count and the time seniority comes from when one started karate training. If this Sensei is mature enough to be respectable and able to give you a life guidance is totally another matter.
Another person asked, “My sensei is only Nidan. I thought a real sensei must be Yondan and above. How should I consider him?” My answer is “He is your Sensei.” Anyone who is in front of a class and teaching is a Sensei regardless of his/her dan rank. Whether he/she is qualified to teach (if he/she has a teaching certificate) or not is another matter. Besides, having a teaching license does not automatically make this person a good Sensei.
I know a Nidan who has been that rank for over 30 years. His training and teaching experiences probably exceed those by a younger Yondan. I have seen poorly planned instruction by many senior (7 dan and 8 dan) instructors.
The key is if the instructor is enthused enough to share the knowledge and the skills he own. If you can learn something from him or her then he/she is your Sensei. If you are not learning little from him or her, you can always quit the dojo and find another dojo or Sensei.
We expect our Sensei to be more than someone who teaches how to punch and kick. This is true because Karate-do is more than just punching and kicking. You are lucky if your Sensei can teach you more. Can we expect this from a Sensei of 25 or 30? Some may be very matured and have many years of Karate training but most of them may be too young and lack those qualities. So do not have a wrong expectation from a young Sensei. His minimum obligation as an instructor is to be able to teach the Karate techniques. This means he can explain and demonstrate those techniques.
On the other hand, not all senior or old Sensei have the qualities and qualifications either. Maturity and wisdom do not necessarily come with the age. Many of them get out of shape. If an instructor is too overweight and out of shape to demonstrate the techniques, I do not consider him as a responsible instructor.
I like what Musashi said some hundreds of years ago. He said everyone other than himself was a teacher to him. I follow his concept. My original Sensei (Sugano and Asai) may be dead and gone. I believe my current Sensei is everyone who comes through my life whether he is in martial arts or not. I want to learn something (good or bad) from everyone and all experiences in my life. That is my philosophy and I am not expecting the readers to agree or accept it.
Conclusion
How you select a Sensei is totally up to you. Each of us has different expectations and objectives from our training. I hope you have a Sensei who you are happy with. If you do not, I hope you will find one you will be happy with and can learn a lot from.
If you are a Sensei in your dojo, The minimum obligation you have is to teach the correct Karate techniques. This means you need to be in shape so that you can not only explain but also demonstrate those techniques you teach. In addition, I hope you try to provide more than the karate techniques. Many of your students are expecting this.
Article by by Shihan Kousaku Yokota. 8th Dan Shōtōkan Karate – Asai Shotokan Association International (ASAI)