No, for me, I just wouldn't care, at least not much. It wouldn't be the thing I base the judgement on, unless perhaps it was really, really close to the borderline anyway. i.e. I was going to vote for candidate A, but then I heard about some really bad behavior when he was 17. I can't see myself changing to candidate B because of that unless I was pretty undecided in the first place.
It sees to me that in your hypothetical, one person was caught at 17, punished, publicly repented in the face of social pressure, and went on to lead a decent life afterwards while a second person got away with it, never did anything like it again, probably regretted it but because it never went public no one ever knew about it, but then it was put into public view 30 years later, and we should hold it against him, but not against the one who got caught at 17? No, I'm not seeing it.
If we were to discover that Kavanaugh actually raped someone at 17, maybe it would influence me, a little. In the situation that exists right now with judge Kavanaugh, if we were to discover that he did that, it would influence me, only because he denied it today. If we were to subsequently discover that, well, yes, it did happen, then that would mean he is lying today, and we would be judging him on his behavior today, not when he was 17.