18 years

I remember this date well: September 17, 1991.
I do not think that more than one or two people checked that version. For
it was necessary to mess around with installing a special compiler, select
empty partition to use for booting, compile my kernel
and run the shell. And besides the launch of the shell, there was nothing special to do.
It was possible to print the source code - only 10,000 lines, i.e. less than a hundred
pages, if printed in small print. (Now there are already about 10 million
lines.)

Linus Torvalds, excerpt from the book "Just for fun"

Exactly 18 years ago, Linus published the Linux kernel source.