The citations in the OED trace the printed history of words and their meanings. I agree that these are just the authenticated usages, and they may be extended as more information is collected. The 1987 supplement added many earlier and later appearances of many words, and many more were added in the latest edition. Until new authenticated information is found, however, the rest remains speculation. The OED is the benchmark.

New information was found in this case, see the current thread on the English slang word 'jankers'. Thread 59694: jankers . The latest edition I have puts the word back to about the First World War, but seemingly it was already being used in the early 19th century. This example may have been added to the latest edition.

Anyone may submit new information, with the support data and reference, to the OED.