The idea of prehistoric garments is typically highly utilitarian, and I usually see patchworks sewn together as the depiction of such. Clearly, fashion existed in Ancient Egypt, and from there on, it served as an important piece of culture. So at what stage was humanity prosperous enough to invest time in ornamentation en masse? Am I wrong in entirety, and did ornamentation arise long into prehistory?
EDIT: I have posted this question to /r/AskAnthropology as well. Link here
The dawn of time.
The famous "iceman" mummy had tattoos and he died in 3300 BCE.
Some of the earliest human burials we've found contained grave goods and I'd argue that was fashion of a sort.
Finally, I think this is (and isn't) an answer:
Chimpanzees are capable of fashion.
You might want to x-post this to /r/AskAnthropology since your question seems to fall more into their domain.
Like the others have suggested, I believe that some idea of fashion has existed for many years.
That said, Susan Mosher Stuard (an excellent scholar) has argued that the fourteenth century in Italy was the first "Great Age of Fashion" during which access to luxury goods was prevalent throughout most of society. It's really not worth it to get into the nitty gritty of her argument (there's a lot there about sumptuary laws), but if it's something you're interested in, it's fairly short and an enjoyable read.