Old ale is a term commonly applied to dark, malty beers in England, generally above 5% abv, also to dark ales of any strength in Australia. Sometimes associated with stock ale or, archaically, keeping ale, in which the beer is held at the brewery.
Historically, old ales served as a complement to mild ales, and in pubs of the era, typically the landlord would serve the customer a blend of the sharper stock ale with the fruitier, sweeter, mild ale to the customer's taste. In London, especially, the aged ale would take on a tart note from a secondary fermentation with brettanomyces yeast, which was present either in the pitching yeast or in the wooden equipment. Because of the time required for the aging process, some investors would buy mild ale from brewers, age it into old ale, and sell it at a higher price. Eventually, brewers began to keep some beer behind at the brewery, age it themselves, and sell it to the pubs. In some cases, old ale was a blend of young and old. The "stock ale" was the brewery's very aged ale and was used to inject an "old" quality, and perhaps acidity, to the blend.