According to this, the first was Boot-Root from Torvalds himself in 1991. The oldest that are still around are Slackware (July 1993) and Debian (Aug 1993).
This is gonna blow your mind. There this thing called the internet, and people put loads of information on it. You can access this using “websites” called “search engines” that index all the content.
Slackware was the first real distro, which means you could reasonably expect to get to a bootable state by following the manual, and have a useful system out of the box.
And it's the oldest that's still around.