Without digging into boxes of old stuff, the oldest thing I have still in some active use is the EF 50mm f/1.8 with a 1988 code:
Interesting that it was a kit lens with the SLR I bought around 1990.
While I say still in some active use, it's really been supplanted by the 40mm pancake. As anyone using the old 50 knows the AF motor makes the sound you hear when you crank up an old castle drawbridge -- and it's not much faster either!
Same. The. 50mm f/1.8 with the metal mount is much sought after.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye