I've occasionally added a 'Best Answer' very belatedly to a thread but I can't recall one ever having been from as far back as that though.
My own BA score quite often seems to go up for no obvious reason. I look back through my 'Best Answers' for the past week or two but I see nothing that I didn't already know about, so people are clearly adding to them to my posts from quite a while back.
I'm sure that the recipient of your belated BA will dance an Irish jig when she sees that her score has gone up though ;-)