My BYO 15th Anniversary Ale — which I brewed in March of this year, and the recipe for which was featured in the September 2010 issue of BYO — is now about 7 months old. Despite coming in at a calculated 12% ABV, this beer conditioned rapidly and was in drinkable shape after only about 6–8 weeks. I've been sampling it off and on since then as it continued to mature. Now I would say that it has hit a plateau.
The fully conditioned beer isn't that much different from the younger version, but there are a couple differences. The phenolic edge from the Trappist High Gravity yeast has mellowed just a bit and malty character from the Vienna, Munich and melanoidin malt has come forward. It wasn't out of balance before, but now it tastes more integrated, with the malt flavors, hop bitterness and yeast-derived characteristics all mingling together. The maltiness is the most noticeable character, to me at last, but it doesn't overwhelm the other aspects of the beer. Although, it's a big beer, it's now very sweet or chewy; in fact, it's almost dangerously drinkable, given the alcohol content. To me it almost tastes as if it's the strength of a doppelbock (7–8%).
I suspect that this beer will hold in good condition for quite some time — which is good, since its really yummy (if I do say so myself) — and may post further updates on its aging.