Joho quotes tinyapp.org on the oldest domains on the internet still in use. The oldest 15:
03/15/1985 SYMBOLICS.COM04/24/1985 BBN.COM05/24/1985 THINK.COM07/11/1985 MCC.COM09/30/1985 DEC.COM11/07/1985 NORTHROP.COM01/09/1986 XEROX.COM01/17/1986 SRI.COM03/03/1986 HP.COM03/05/1986 BELLCORE.COM03/19/1986 IBM.COM03/19/1986 SUN.COM03/25/1986 INTEL.COM03/25/1986 TI.COM