That the event happened (if it happened at all) 30 years ago does in fact matter -- it bears on whether we can prove it happened or not. Memories over that period of time are very unreliable and physical evidence goes missing or changes. Kavanaugh was a minor or young adult at the time and has done a lot of growing up since. If doing stupid shit while young was a disqualifier, Robert O'Rouke would not be running for Senate.
Some things are bad enough that you don't get a pass. Murder is on that list. Forcible rape probably is, too. But horseplay and mild groping at a drunken party as a minor probably shouldn't be. If it had happened last year, it would demonstrate something about Kavanaugh's lack of character in the present. But if it happened at all, it happened so long ago that it has little to say about the man today.