I currently have 12 cases of beer, cider, and mead in my basement. Some of the commercial stuff has been down there for upwards of 8-10 years, some probably more. I believe it's all still good, otherwise I would have dumped it out already.
Regarding homebrews, I don't usually age it for that long. Only if it makes sense, like for a barleywine or Belgian strong I might keep it around for 3 or 4 years. Otherwise I might keep some for a couple years but by the end it is usually oxidized pretty bad so I'll either finish it off or dump it. Whether you are bothered by oxidation or not is a matter of personal preference, and differs a lot from batch to batch. I kind of do care, but I'm also pretty much the only person drinking these beers, and am NOT a heavy drinker at all. I've not had problems with it going sour for many many years, just oxidation is the big thing, and loss of hop character.
I once made a Scottish 80/- ale, all in bottles, that for whatever reason tasted as great several years later as the day it was first bottled. I hung onto a few bottles for I think like 7 or 8 years and shockingly it never went off or oxidized, was always great. I love that recipe. I need to brew me some more of that. Maybe play with the hops a bit next time, aiming closer to Wells Bombardier or Coniston Bluebird (yes I know those aren't Scottish but humor me).