I recently opened a bottle of English barleywine that I brewed 9 years ago. I have to be honest, I am really surprised with how well it has held up over the years. I think it's been at least 4 or 5 years since the last bottle I opened, and the only major difference I noticed was that the hop bitterness has faded significantly. Unfortunately, it still has a bit of a husky astringency that has faded slightly, but not to the extent I had hoped. I'm glad I decided to bulk age this in a keg and bottle from there rather than bottle condition, because there is none of the autolyzed soy sauce flavor that I get from old bottle-conditioned beer. There is certainly some oxidized character, but (probably because this beer attenuated quite well 1.142 OG > 1.024 FG) it's not a real cloying caramel/raisin/sherry note; it's more like port and dried cherries. I'm looking forward to checking back in on this one next year for its 10th anniversary.
If you haven't brewed a big barleywine for aging, you should give it a try. Your future self will thank you. And if you have one tucked away in the cellar, give it a try if you haven't checked in on it recently.