According to my (possibly inaccurate) notes:
NetBSD checked in 1993
Message-ID: <alpine.NEB.2.20.1803211456560.25928(a)t1.m.reedmedia.net>
Revision 1.1, Sun Mar 21 09:45:37 1993 UTC (25 years ago) by cgd
http://cvsweb.netbsd.org/bsdweb.cgi/src/sbin/init/init.c?rev=1.1&conten…
"Today is commonly considered the birthday of NetBSD. As far as I know,
it is the oldest continuously-maintained complete open source operating
system. (It predates Slackware Linux, FreeBSD, and Debian Linux by some
months.)"
-- Dave