It’s a pattern basically. If you buy something today , expect it to be worth something in 25 years. If you find something that’s over 25 years old and it’s not worth much chances are it’s never going to be sought after. There are exceptions to every rule, outliers, freak happenings, but 25 is a good number to gauge everything as a whole.
Its 2020 and 25 years ago is 1995. Many action figures, vinyl records etc are seeing the spike. Staying ahead of the curb may include shopping for stuff that was brought to market in the late 90s, early 2000s. So your wait time isn’t as long.
edit to add: video games and VHS too.