the oldest I've got is manufactured in Nov 1988 (Helium Neon from JDS). It had operated for over 20 years when I bought it and it still works perfectly without any visible degradation.
P.S. Apparently the very first gas laser (now in smithsonian museum) is still in working condition.
My oldest gas-laser is a shool physics teaching HE-NE-Laser of the year 1984.
Manufacturer ist Phywe and the power is 5mW.
See the pictures from today: