Aging past the "Best by...." date?

I just stopped at a bottle shop in another state that I don't get to very often and picked up a 750 of Unibroue's Don de Dieu which I've wanted to try for a long time. I get home and am lovingly ogling the bottle when I see that the Best By date was 4 months ago. Unibroue says that their beers age nicely but the ones purchased along with the Don, La Fin and Trois Pistoles, had best by dates in 2014 so I assume the bottle of Don I got was past the time Unibroue brewers would have considered it to be in it's prime. Question is, should I drink it right away because it's starting to go downhill or can I save if for a few months to have with my brother when he comes for a visit which was my plan when purchasing?

 

Well, I opened the Don and now I'm regretting not saving it until my brother comes to visit. It's 4 months past the best by date so it's got at least a couple of years since bottling and it's just fantastic. Still got good carbonation and I taste no oxidation. I think it's as good as La Fin. At least now I know that they can go past that best by date and I imagine the dark ales would age even better. I don't cellar anything intentionally but it's useful information