A best before just means that after X amount of time it's still, harmless/efficacious/palatable.
Let's take a drug, any drug, the authorities test the drug for how many yrs? Let's say 10yrs, after which the efficacy is 85% as opposed to 90% at day one, their cut off is actually 80%, do they hold back for another 10yrs? Well given the cost of testing probably not, they will just say best before 10 yrs because their testing has shown efficacy within that period. Note the drug would probably be beneficial for the next 20 or 30 yrs, but who will wait that long, and how much money would it cost to continually test/monitor. How many yrs would pass and how much money spent before a "use by" date could be established?
To answer your questions:
Depends what you're talking about.
Change: good and/or bad.
14yrs ish maybe.
2000 to 2001 ish, Kegged (19ltr) an Imperial stout, and a barley wine (3 stouts, 2 barley wines) put them under the house, subsequently found my wife in bed with a bricklayer, mmm you did ask what happens over time did you not?
Luckily had two houses at that time so removed myself to the tenanted property (little ex govy job)
2004 with my new partner (now and still is my wife) went to PNG (ironically both of us wanting to escape the "Oh I'm sorry" from our previous partners)
2011 returned to Aus
2012 got a msg from my ex "can you come and clean out your s**t from under the house?
Retrieved numerous kegs and milk cartons full of bottles, the snails had eaten any chance of identifying the bottles, but the 5 full kegs I remembered, just back from overseas takes a while and many bottles were tried, not many were palatable, with about 50% undrinkable.
2014 Off to PNG again, NFW are those kegs going under the house again, so I dug out my old faithfull (2 tap fridge that I brought with me in 1985 from the UK) for the "farewell party".
Now! fully expecting the whole lot to be chucked: 1 barley wine was undrinkable, 1 Imperial stout was borderline undrinkable (my palate) but the other barley wine was considered pretty good (I would give it a 6 ish) the other 2 stouts however were bloody marvelous.
The party was Saturday, Sunday morning had bodies all over the place (Canberra summer) but the highlight for me was people showing up on Monday afternoon asking if there was any more of "that stout" left. And they weren't traditional stout drinkers.