I see Wikipedia reckons that “According to Martyn Cornell, ‘no historically meaningful difference exists between barley wines and old ales’.” Do I think that? You’ll be unshocked to learn that my beliefs are actually considerably more complex.
One problem is that in the real world, beer styles such as Burton Ale that have been called “barley wines” have also been called “old ales”. And milds. Another is that, as Michael Jackson pointed out in the 1970s, “barley wine” seems to be a name given to some very different beers. If Thomas Hardy’s Ale and Gold Label can both be called “barley wine” despite being utterly different beers, then we have a serious problem in defining “barley wine” as a style.
In fact I don’t believe there is actually any such meaningful style as “barley wine”, despite the BJCP attempting to draw up rules. Indeed, just to show how arsey such attempts at categories are, if you click on that link you’ll see the BJCP calls Fuller’s Golden Pride a barley wine and Fuller’s Vintage Ale an old ale, although they’re actually the same blahdy beer, the latter version being bottle-conditioned, the former not. And while Ratebeer and the BJCP says Thomas Hardy’s is a barley wine, Beer Advocate calls it an old ale. So: make your minds up, Yanks.
While you’re deciding on categories, however, I have to tell you – I’ve come to the conclusion that, as well as barley wine being effectively meaningless (it doesn’t even mean “any strong beer”, since no one seems to categorise Imperial stout as barley wine), there’s no such style as “old ale”, either. Not historically, anyway. From which it follows that (and this really will kick the hornet’s nest) there’s no such style as “mild”*.