I'll take long term as meaning more than 12 months. That is what you meant, isn't it, Alan?
The following is a quote from "A Treatise on Adulterations of Food" by Fredrick Accum, published in 1820:
"OLD, OR ENTIRE; AND NEW, OR MILD BEER.
IT is necessary to state, that every publican has two sorts of beer sent to him from the brewer; the one is called mild, which is beer sent out fresh as it is brewed; the other is called old; that is, such as is brewed on purpose for keeping, and which has been kept in store a twelve-month or eighteen months."
He's talking specifically about Porter, in case you were wondering. As Accum went round the large London Porter breweries (and took samples of beer for analysis) I think he knew what he was talking about.
This next quote is from "The London and Country Brewer" of 1736:
"A particular way of Brewing strong October Beer.
There was a Man in this Country that brewed for a Gentleman constantly after a Very precise Method, and that was, as soon as he had put over all his first Copper of
water and mash'd it some time, he would directly let the Cock run a small stream
and presently put some fresh Malt on the former, and mash on the while the Cock
was spending, which he would put again over the Malt, as often as his Pail or Hand-bowl was full, and this for an Hour or two together; then he would let it run off intirely, and put it over at once, to run off again as small as a Straw. This was for his _October_ Beer: Then he would put scalding water over the Goods at once, but not mash, and Cap them with more fresh Malt that stood an Hour undisturbed before he would draw it off for Ale; the rest was hot water put over the Goods and mash'd at twice for small Beer: And it was observed that his _October_ Beer was the most famous in the Country, but his Grains good for little, for that he had by this method wash'd out all or most of their goodness; this Man was a long while in Brewing, and once his Beer did not work in the Barrel for a Month in a very hard Frost, yet when the weather broke it recovered and fermented well, and afterwards proved very good Drink, but he seldom work'd, his Beer less than a Week in the Vat, and was never tapp'd under three Years."
It's discussing the private brewing October Beer, a strong beer meant to be aged that many reckon to be the forerunner of Barley Wine. Three years before tapping - sounds like long-term ageing to me.
At Guinness, the Export Foreign Extra Stout (FES) had a long ageing:
"FES at this time was only brewed in the 3 winter months January to March when DS and SS sales were lowest. It was due for sale in the 12 months of the following year (January-December), on average 15 months old. . . . . Due to the prolonged storage of up to 2 years in vat before sale, FES brewing had to be limited due to the available vathouse capacity and the volume brewed well ahead of sales."
"A Bottle of Guinness, Please" by David Hughes, pages 71-72.