If you mean in a marital or dating relationship, then age matters if it matters to YOU (or to him or her).
If you mean in a friendship, age should not matter once you are both adults, and no, the older person does not always have to be the mentor. I have friends that could be my mother and I don’t look at them as mentors. I respect them for their age with which may come knowledge, but I don’t look up to them for answers or quidance.