Comparing what I remember to the version history on the wiki, I think it was back in 2013, somewhere in between the 1.0 release and the 1.037 beta. Not quite from the very beginning, but I was definitely there fairly early! For me, cookie clicker has been one of those games where I'll play it a bunch, then not touch it for a couple years, then come back and do a new run to appreciate the new features. I'm the same way with minecraft. It's really cool to have a game you can come back to every so often and see how it's grown and changed while also getting a bit of that sweet nostalgia. Despite everything, you can still boot up your computer and click some cookies.