we actually did a tasting recently. My father in law has celebration ale dating from 04 to present. (we buy 32 cases a year) in a recent tasting...we have found that 3 years is the magic number. the 04 and 05 were both still good, but not as enjoyable as the 06. the 07 was better now than when it was new and the 08 is a little better now than when first purchased. this is based of of the average ratings from 7 of our close friends/family that enjoy great beers.
Keep it in the dark (we keep it in a 1800's farmhouse basement) and it will be fine)