I found these two street signs in Wauchula, FL. The first set has been supplemented with newer signage while the second was not. Makes you wonder how these two sets survived, couldn't find any others. Also notice the exposed brick and old looking pavement and curbing with the second set.
Set 1 (https://www.google.com/maps/@27.5462799,-81.81867,3a,75y,322.71h,86.44t/data=!3m6!1e1!3m4!1sSOqwj7tHwJE6l6_fxyxnug!2e0!7i13312!8i6656)
Set 2 (https://www.google.com/maps/@27.5458732,-81.8206786,3a,75y,328.97h,72.58t/data=!3m6!1e1!3m4!1skAru2dOvwuT4WV9pnfC0tw!2e0!7i13312!8i6656)
I can't think of any older signage I've seen in my town. With the hurricanes it seems they get blown down and have to be replaced every so many years, along with the constant trasition to larger and taller street signs.
My street used to have two bridge signs that appeared to be from possibly the 70s or 80s but they have since been replaced. All signage near me now bears the label on the back with the pop outs for inspection dates. Older signage had the county stamp instead. There are some older reflector markers but that's all I can think of.
It seems "Neighborhood Watch" and "Children Playing" signs tend to be older though in most areas.
On Coxcomb Dr. in Astoria, OR, there's a very old "Sharp Curves" sign: Google Earth link (http://"https://earth.google.com/web/@46.18218888,-123.8254101,108.99105072a,0d,59.99999y,123.49132398h,81.55248562t,0r/data=IhoKFjB6bFdIeEZ1QXZmUlA0N013MXV3ZEEQAg")
(https://i.imgur.com/58RVm4o.png)
There used to be a identical looking sign before it that said "Drive Safely" but it was removed a few years back.
There's also a white/square "DEAD END" sign on 8th street in Warrenton, OR: Google Earth link (http://"https://earth.google.com/web/@46.15861076,-123.92820788,5.27029085a,0d,15.92696726y,218.72237742h,88.7019681t,0r/data=IhoKFmpROFpGNWZwXzVtUm5zZUh3RXhfSWcQAg")
(https://i.imgur.com/r97UkJL.png)