The size range of most adult women’s clothes was once referred to as “misses” sizes, as in “young misses”. So a size 12 (before vanity sizing existed) was actually intended to fit an average 12 year old and the size numbers referred to the approximate age of teen that could be usually be expected to wear it. There was no size 0 back then, misses clothing usually went from 12 up to 20, although the range sometimes started at 10. After that, a woman moved into “woman’s” sizing and the next number up was 40.
“Vanity” sizing started to appear in about the mid 80s and every manufacturer had their own idea of what a size 12 was supposed to be. This is when I first noticed that off the rack sizes no longer matched up with sewing pattern sizes.
Here’s an example of a sewing pattern size chart from the 1950s. All the pattern companies used standard sizing, with minor variations.
When that size system first started being used, size zero would have been for a newborn infant. Actually, size 0 didn’t even exist then.