Orwell's assessment agrees with that of Howard Haycraft, who in Murder for Pleasure: The Life and Times of the Detective Story (1941) coined the term "Golden Age of Detective Fiction" for the post-1918 style of English-language detective stories. For Haycraft, detective fiction has to have a detective (it is not merely about a crime or a puzzle) and does not necessarily have a murder - for example, he heaps praise on Poe's murder-free The Purloined Letter because of the ingenuity of the solution, even though considering The Murders in the Rue Morgue to mark the beginning of the genre.
While establishing a definite era start date is clearly futile, it's still true that this corpus of stories is an identifiable "something". The writers (Christie, Sayers, Crofts, etc.) generally knew one another, and collaborated in defining a genre. They formed clubs. They wrote criticism and polemic. Perhaps most importantly, they were collectively prolific, writing enough stories that they quickly established a sense of genre cliches (to be avoided or subverted) and figured out what the reading audience most wanted.
S. S. Van Dine, a pseudonym of Willard Huntingdon Wright, wrote a set of genre 'rules', somewhat provocatively, including this No. 7 ("S. S. Van Dine Sets Down Twenty Rules for Detective Stories", The American Magazine, September 1928):
There simply must be a corpse in a detective novel, and the deader the corpse the better. No lesser crime than murder will suffice. Three hundred pages is far too much pother for a crime other than murder. After all, the reader's trouble and expenditure of energy must be rewarded. Americans are essentially humane, and therefore a tip-top murder arouses their sense of vengeance and horror. They wish to bring the perpetrator to justice; and when "murder most foul, as in the best it is," has been committed, the chase is on with all the righteous enthusiasm of which the thrice gentle reader is capable.
Likewise, publisher Dodd, Mead & Co. had a "test" in the 1920s and 30s for whether submitted manuscripts were acceptable for their mystery imprint, featuring rule G (reproduced in Watteau's Shepherds: The Detective Novel in Britain, 1914-1940 by Leroy Panek, 1971, Appendix I):
On the gravity of the crime depends, in large measure, the reader's interest in the identity of the criminal. For this reason the crime should be murder or potential murder.
Other 'rules' were promulgated by writers and critic on both sides of the Atlantic. Panek suggests that the point - to the extent that these are serious attempts at defining a genre, as opposed to playful self-critique - is to identify the central appeal of the detective story per se. Because the genre is about investigating a crime by intellectual means, where all the clues are on the page, the crime needs to be one which is explicable but also non-obvious. Thus, it is not "allowed" to be a case of a crime of mad passion, a gang conspiracy, a suicide, etc. - it needs to be premeditated in order to have the requisite complexity. All the justifications cited by Van Dine also seem reasonable.
In any case, whether justified or not, this became a hallmark of the genre, to the exclusion of stories which didn't fit the bill: for example, thrillers where the plot resolution doesn't come about as a result of investigation and deduction, or ones where the crime is unsolved at the end. As ever, there are certainly classic detective tales from this era that break many of the rules. Raymond Chandler would say (and did, in "The Simple Art of Murder", Atlantic Monthly, December 1944) that the genre rules encourage unsatisfactory writing, where the horror of murder is lessened by making it part of an elaborate puzzle-box, where justice wins in the end, and characterization is often absent.
This brings us back to 1918 as a start date. The point of this cutoff is that it's the end of the Great War, after which the public appetite for stories of all kinds tended towards the entertaining and comforting. There are certainly earlier stories with Golden Age characteristics, and later ones without. But post-War societal changes created the right conditions for 'cosy' tales of ingenuity in the pursuit of justice, where there may be a crime, but that aberration is swiftly and inevitably corrected.
