balls
It depends on the school and the teacher in all honesty.
I've had colleagues who are M6/UPS who were any of the following: poor mentors, couldn't be bothered, only interested in creating mini mes, still insist that a good lesson caters for learning styles, generally promotes in trainees and NQTs fads from 10/15 years ago, tells trainees they need more "buzz" in their lessons, they need to get the students on side before expecting too much work out of them, lesson feedback was more about talking about tasks than pedagogy.
I've worked with colleagues on M3/4 who were fab mentors, didn't claim to have all the answers, were actively involved in their own CPD, good at promoting reflection, offering advice, signposting to relevant colleagues, spends time keeping up to date with research.
The M3/4 colleagues may not have 8 years of experience, but they would make a better mentor.
Obviously, there's some new staff who would be utterly unsuitable to mentor (thinking about a colleague who's M1/2, has weaknesses in their own practice but is seeking to run before walking), and there's some amazing experienced staff who do all the things that the M3/4 person above does and has the benefit of time as well.
I'm not entirely convinced that time served should automatically be a criteria for taking additional things on.