oldest tube(s) in your system

Most of my 6sn7's and VT231's are from the 40's and 50's. National Union, Ken Rad, Sylvania, RCA, Raytheon, Tung Sol and no doubt others. Lots of nice vintage tubes, a few newer ones from Amperex and Electro Harmonix but mostly favor the ones from the 40's and 50's. 

I also have a group of 7n7's which I haven't even tried yet. I think they are 40's and 50's, all are metal base tubes. 

For 12AX and !2AU7, I mostly prefer tubes from the 50's, Sylvania GB Gold Pin 5751's and also Amperex Bugle Boys. 50's and 60's I guess?

I used to run RCA mono-plate 2a3s made before 1936.  Those are good sounding tubes.  A friend runs extremely rare and old 252 meshplate anode tubes made in the 1920’s that pre-date the 300A.  He rarely plays the amp because those tubes cost well north of $30,000 a pair.

I run some globe 866 mercury rectifier tubes, probably made in the 1930's.