The OED identifies this metonymic expression—which describes an early stage in an event or process, often implying too early, or premature—as “chiefly British” and finds a sixteenth-century citation from Sir Thomas More: “She telleth hym then that it is but early dayes, and he shall come tyme ynough.” It also shows up in Samuel Richardson’s 1740 Pamela (“’Tis early Days with Pamela, and she does not yet think of a Husband”) and frequently in the late 1700s and early 1800s, usually with the word “yet,” meaning “still,” at the end.
I should note that Americans have always referred to beginnings as “the early days of” something. It’s just that they only started saying “it’s early days” around 1980, as Ngram Viewer shows.
An early New York Times use came in 2001, when restaurant critic William Grimes wrote about the staff at a venerable French restaurant, after a change in management: “It’s early days yet, but I think they realize that Lutèce has turned a corner.”
By now, it’s common enough to be viewed as a cliché or—as the American tech writer Molly White observed in 2022—an excuse. White wrote that when she points out some of the shortcomings of blockchain currency (which has been around since about 2009), she’s often told “It’s early days.” However:
“So this raises the question: How long can it possibly be ‘early days’? How long do we need to wait before someone comes up with an actual application of blockchain technologies that isn’t a transparent attempt to retroactively justify a technology that is inefficient in every sense of the word? How much pollution must we justify pumping into our atmosphere while we wait to get out of the ‘early days’ of proof-of-work blockchains?… The more you think about it, the more ‘it’s early days!’ begins to sound like the desperate protestations of people with too much money sunk into a pyramid scheme, hoping they can bag a few more suckers and get out with their cash before the whole thing comes crashing down.”
Possibly creating confusion is the existence of another British expression (which hasn’t penetrated to the U.S.) with a similar sound and meaning. The website World Wide Words offers a 2010 quote, and then a fascinating history:
“We’ve got to make sure we don’t concede, especially early doors, but I think it’s definitely game on if we score first.”—Sporting Life
Why footballers, commentators and fans say “early doors,” when “early” or “early on” would work just as well is probably due to Big Ron, otherwise Ron Atkinson, a well-known television football commentator, a former player and manager now regarded as one of the characters of the sport.… However, my memories of the phrase go back to Brian Clough, a rather more famous football manager, who is on record as using it in 1979. …
In the days before liberalisation of hours, pubs would reopen for the evening at 5.30, just in time for a quick drink after work and before going home. An early-doors beer would be one grabbed as soon as possible after opening time….
We’ve actually got to go back well over a century to find the true origin… Then as now, a last-minute crush usually developed at the entrances [of theaters] just before the performance started, with the street outside crammed with vehicles…. Around the 1870s, the idea grew up of charging a small premium to members of the audience who were willing to arrive well ahead of the crowd; in return, they were allowed to choose their own seats in unreserved areas — the pit and the gallery in particular. This could be a considerable advantage, as sightlines in those areas were often poor and interrupted by pillars…
The system continued into the twentieth century and became very well known:“The park-keeper eyed him; thought better of the bitter words he had contemplated; contented himself with: ‘Funny, ain’t yer?’ ‘Screaming,’ said George. ‘One long roar of mirth. Hundreds turned away nightly. Early doors threepence extra. Bring the wife.’–Once Aboard The Lugger, by Arthur Stuart Menteth Hutchinson, 1908.
It was recorded by G.K. Chesterton as a First World War battle cry by Tommies going over the top to attack the enemy (‘If they had only heard those boys in France and Flanders who called out “Early Doors!” themselves in a theatrical memory, as they went so early in their youth to break down the doors of death.”). Theatres seem to have stopped the early-doors practice in the early 1920s.