Approximately when kanji was introduced to the already-existing Japanese spoken language. Fourth or fifth century AD possibly, but certainly by the mid-seventh.
The post WWII kana overhaul was essentially an official codification of kana practices that were already in common usage, plus the introduction of more standard spelling to represent how words were actually being pronounced. The hentaigana had already been dumped in 1900, though (along with the addition of ん to the canon set, which was previously a hentaigana for む).
… When they invented fonts. Around the same time as the printing press.