The debate about how senior a mentor should be to a mentoree is a tough question.
Does seniority make that person any better than someone else at anything?
Does their seniority reflect their experience, their luck or a combination?
Is mentoring better first established through the lens of seniority vs. experience?
Does what we seek from mentoring require hierarchical mentor or an experienced one?
I think many of us approach the seniority of a mentor in two ways:
1 - with a secret hope that, like a fairy godmother, the senior person will personally champion our cause and shepherd us to stardom (like an apprentice or favoured pupil)
2 - with the assumption that the because the person has seniority in an industry, that they are self-aware
Sometimes, #1 can come true. But not often. Mentoring - both sides of the relationship - involves hard work. Good mentors often have several mentorees, leaving the individuals to find and define success for themselves. Mentorees often have several mentors - who can't all take you to the top of their ladders. So #1 isn't often an ideal way to approach mentoring.
#2 still puts the onus on the person seeking mentoring. When you select a mentor, if seniority is your first criteria, you may not be clear on what you're actually seeking. You may be stuck on the #1 dream :-) A good mentor is self-aware in order to be able to share why they are successful - or at least self-aware for the areas you wish to explore.
I'm not saying that seniority isn't part of your consideration in selecting a mentor or agreeing to take on a mentoree. It just should not be the first consideration?