I think you, and others, are missing the point.
No one would disagree that most of us did and said things as teenagers that we either now regret or time shows in a different light. Not all of us though aspire to become Prime Minister or behaved in quite the way Farage is alleged to have done. That behaviour being unacceptable even then.
However that’s not the most important point. It’s the way he has responded to the allegations that is of most concern. He hasn’t tried to distance himself from them and reassure that his past errors were just schoolboy pranks. It leaves the impression that the underlying attitude is still there. Something he confirms on occasions with other things he says, or doesn’t say.
I know he needs to keep a core group of resolutely racist supporters onside but he risks alienating a much larger group who aren’t.