I'm currently going through the thousands of Kodachrome slides that my father left at his passing. Most are processed in Canada, but a fair few are USA processed.
The oldest I have found are probably from 1947 and are badly faded. Most of the rest (so far) are in excellent shape.
Most have now hard to read printed dates on the mounts, but some have embossed dates.
And then there are the strange outliers, like Kodacolour transparencies, or non-Kodak processed 127 Ektachrome or what I expect are duplicate transparencies where there are either no dates or dates that are later than makes sense.
Word to the wise - if you can do this before your parents are gone - do so!