Maybe I'm totally misremembering, but I'm sure that people were calling the Mega Drive and SNES retro pretty much as soon as the Playstation came out. I can say that I definitely saw people calling Final Fantasy 6 and Chrono Trigger "retro RPGs" by the early 2000s since the ROM sites I used called the Mega Drive and SNES "retro" consoles. This means people were applying "retro" to things not even a decade old.
This kind of gets to my issue with making "retro" defined by years-past rather than an era in and of itself. I know it starts getting into the weeds, but to me there is a massive difference in games before the mid-to-late 90s and afterwards. I mean, Doom and Half-Life are only 5 years apart but they might as well be a lifetime for how different they are.
This is why I put gaming into three main categories: Retro, Pre-modern, and Modern. The Playstation and N64 changed things, but that change wasn't fully manifest until a few generations later.
Though I am sympathetic to marking the end of "retro" to be the death of Sega as a major player, since even Nintendo had to take a "if you can't beat them, join them" mindset eventually.