>>280330218
>'20s
Arguably true for shounen and shoujo
For the most part dedicated manga magazines before WWII were unisex but clearly targeted towards boys, and this provoked the surge of separated magazines for girls, and in turn this cementified the male target for the original magazines
Those two categories became huge in the '20s and '30s, yes, but I would argue that the post-war guidelines that were enforced and the need to rebuild the publishing industry from the ground up formed the basis of what a shounen / shoujo magazine content became from that point onward in a more relevant way to today compared to the prewar view of manga.
Seinen and Josei became explicit editorial categories in the late '50s / early '60s