The answer is there no set definition or universally accepted rule and never will be. Realize that someone who is 18, 25 or 30 will see past time differently than someone here who is 50 or 60. 1985 may seem like yesterday to some of us, but it's almost a generation before someone was born.
The simple way is to catalog by years: Pre-WWII, 1970-80s Topps, etc.
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