Yet here's evidence of Porter being aged in the bottle:
"At the Wine Vault in the West Wing within the Exchange, WINES and SPIRITS of different kinds, are to be sold, in wholesale, and at the following prices in retail, for ready money, viz.
Claret wine, upwards of eighteen months in Bottles, at 2s. 6d. and 3s. per chopin bottle, and in pint bottles at 5s. 6d. and 7s. 6d. per bottle.
Red port, }
Malaga, and } at 1s. 6d. per bottle.
Lisbon wines, }
Shrub, at 2s. per bottle;
Full proof Jamaica rum, at 2s. 6d. per bottle.
Full proof French brandy, at 2s. 4d. and 3s. per bottle, bottles included in the above prices.
Aquavitae, at 1s. per bottle, or 4s. 4d. per gallon.
Geneva, at 1s. 2d. per bottle, or 4s. 8 d. per gallon.
London porter, Upwards of nine months in bottles, at 3.5d. per bottle, or 3s. 6d. per dozen.
If clean bottles are not sent, when the aquavitae, geneva, or porter is ordered, 1s. 6d. will be added to the price.
Allowance given to such as take quantities.
What is sold of the above articles in bottles, is sealed with the impression A. L.
Commissions from persons exempted from paying impost, will be punctually executed from cellars in Leith, and the impost discounted."
Caledonian Mercury - Saturday 20 June 1772, page 3.
I'd always considered Porter, at least the standard-strength stuff, an almost exclusively draught product. Admittedly, this is an advert from a Scottish newspaper. Bottled beer was much more common, much sooner, North of the border.
Drinkers must have liked bottle-aged Porter or the seller wouldn't have bothered mentioning it. Nine months is a pretty long ageing period, especially if the beer had already been vatted in the brewery. In the 18th century, 6 to 12 months in vats is what you'd expect for Porter. Which means this bottled Porter could have been getting on for two years old. That's a similar length of maturation to a bottled Stock Pale Ale in its 19th-century heyday.