I've had a lot of very old beers. I couldn't list all of them, but...
I've done vertical tastings of Oerbiers and Stille Nacht that dated back probably eight years. Most of them held up well, but they thinned out the older they got.
I had a 2000 Guinness Special Export Stout in 2007 and it was amazing. Ditto these dates and my reaction for Westvleteren 12.
1983 Chimay that was pretty bad.
1978 Saison of some kind that tasted like medicine.
I've had a number of other aged beers, but the most interesting to try were these:
These bottles are 1950s era beers. When opened they made no noise. One is from a brewery in Cleveland, the other Detroit. The minikeg is between 10-20 years old. The bottles tasted like sorghum and penicillin. The Beck's still had some foam to it, but it tasted metallic and medicinal.
If you're really interested in aging beer, here is an interesting article:
http://beersensoryscience.wordpress.com/2010/11/15/chemistry-of-beer-aging/
I've found that some big beers are better after a couple of years, but I'm not a fan of keeping things for longer.