Of all the low-alcohol (<8.0% ABV) beers I’ve aged, I should think that Guinness 250 Anniversary Stout would be the ideal candidate. After 11 months of aging, I can’t say that it did a whole lot for the beer. It certainly didn’t do any harm – it was as excellent as ever. If anything, it smoothed out the already well-balanced character of the beer. The core flavors shifted more toward the dark-toastiness of regular Guinness and away from the crisp cereal-like flavors that distinguished it when fresh. The drinkability hadn’t diminished a bit, though, and I did find it to be overall sweeter and more balanced than regular Guinness.
I’ve had a bottle of Guinness Extra Stout aging for about 4 months and I plan to leave it there for a minimum of two more. I’ll see how that tastes. Guinness Extra Stout has a bit more balls than regular Guinness. Definitely got the liquid pumpernickel taste goin’ on when fresh – hopefully it mellows out a bit. I’m anticipating it becoming more like an Imperial Stout – sweet and malty with a complex coffee / burnt bread flavor and hints of bourbon whiskey, though that may be a bit optimistic. If I felt comfortable leaving it there for two or more years, then that might be a reasonable assumption but at 5.6% ABV, it’s just not strong enough to go that long without turning bitter and sour.