"Old" ales

Hi, Over the years, I've occasionally stashed a Barleywine or Old Ale down in the cellar, thinking "this will be better in a few years". Then I totally forget about it (out of sight, out of mind). Time really flies when you ignore things. Last night, I finally went down and did an inventory. Among other things, I have a 19 year old bottle of Chimay Red, three 15 year old bottles of Thomas Hardy Ale, a bottle of barley wine that the brewmaster at Santa Fe Brewing gave me 14 years ago, two 12 year old bottles of Traquair House Ale, etc... So, my question is... how long is long enough? Or more importantly, how long is too long? I have no doubt that the barley wines and the Thomas Hardy Ale (11.7%), will be good for many more years. But the Chimay is only 7%, and Traquair House Ale is 7.2%. Even though they're all bottle conditioned, I suspect they may already be past their prime(?). I also found a bottle of Samuel Smith Imperial Stout (7%) that was a few years old. I never intended to age that, so I drank it last night. It was still (very) good. Does anyone have any experienceopinion about this? Thanks! Cheers, Tony