Seniority has several aspects:
Total time in the art - A person who has 5 years in the art is senior to one who has 3 years in the art.
Degree of time training in the art - A person who began the art in 1965, laid off for 5 years and then resumed training until 1980 (15 years total time, 10 years training time) is not the same as one who has trained for 15 continuous years. He/she might have "older brother/sister" status but not necessarily seniority.
Rank in the art - a 5th dan is senior to a 3rd dan.
Time in rank in the art - A person who has been ranked 5th dan for 10 years is senior to someone who has been ranked 5th dan for 10 years.
Total time training in the martial arts - A person involved in the martial arts for 40 years is senior to one who has 30 years in the martial arts.
Re seniority it would also be at the senior most practitioner's discretion (usually the chief instructor) who ends up being senior as well, but this would be clearly stated and not implied or interpreted by the students themselves.
A fascinating point is in an art/system/school that has a strong respect ethic, a higher designated numerical rank is not necessarily an indication of being senior. The same is with titles. Pre-WWII Aikido is a beautiful example of this. If you read Stanley Pranin's book Aikido Masters volume 1 you will see many examples of this respect ethic.
Seniority is a very simple matter made complex at times.
This can be done by:
A. Juniors who want to be senior but are not acknowledged as such.
B. Unclear delineation in an art's heirarchy creates a muddled scene.
C. Students breaking off at various times in time line of the art's development and later declaring seniority in the mother art.
D. Students who demand seniority because of aims & goals accomplished.
Physical skills do not necessarily play a part in this as the body ages and wears out. A 20 year old practitioner is going to be able to maneuver better than a 60 year old.
Nor does being in the public eye, popularity or personal PR denote seniority.
Usually it is also determined by the above points and earned respect, not by demand.
Yours,
Dan Anderson