Is it normal for a word's first written use to be decades after people said it Discussion

Bear in mind what survived, as well. Cheap printed material, regional papers and anything ordinary people wrote for each other had terrible survival rates compared to books. The record is biased towards formal registers and that's precisely the register slang isn't in.

Expected, and often large. A first citation is the earliest surviving written use anyone has found, which is a very different claim from the earliest use.

The gap is worst exactly where you noticed it, in slang. Spoken slang goes unwritten by definition until someone has a reason to record it, and the people who wrote things down historically were often not the people using it. When it does first surface it's frequently in a glossary, a court transcript or a novelist quoting speech, all of which mean it was already established.

A rough feel from watching entries get revised: a few years for technical vocabulary that arrives in print immediately, a decade or two for ordinary words, and sometimes fifty years or more for spoken slang.

One useful tell is the tone of the first citation. If the earliest example explains the word to the reader, you're near the beginning. If it uses it casually with no gloss, the word was already common and the real origin is earlier.

Citations also move constantly. Mass digitisation of newspapers has pushed hundreds of first dates earlier in the last twenty years, and amateurs find a good share of them. If a date looks late to you, it might just be waiting for someone to search the right archive.

The practical upshot for reading dictionaries: treat a first citation as no later than, never as exactly when. It's a ceiling on the date, not a birthday.