Note: I take the question from a WW classic perspective, not just U.S. Take it for what it is worth.

Classic is in the eyes of the beholder.

The classic "classic" is clearly the 19th century.
But there are several historical and practical reasons to draw the light "classic" line at 1940, at least for Scott catalogue centric folks.

1) Scott Classic Specialized Catalogue of Stamps & Covers 1840-1940
(Actually the British Commonwealth coverage is to the end of the George VI era- 1952. Makes sense.)

2) "Big Blue"- The Scott International Album Part I covers 1840-1940
This is the first in a series of Scott International albums that cover the world. Part II covers 1940-1949+, Part III 1949+-1955, and so forth. Some choose to concentrate on just collecting the contents of "Big Blue".

3) The "Browns" The most comprehensive WW albums ever published by Scott for WW collectors. One can still obtain volumes that cover the world up to 1940.

4) The "Steiner" PDF albums can be downloaded as a classic subset that covers the years 1840-1940 (-1952 for British commonwealth).
So 1940, whether one agrees or not, tends to be the dividing year.

Personally, considering we are 60+ years past, I like 1950 as the "new" dividing line for "classics".
