Seinen and josei are the adult equivalent to shounen and shoujo. And by adult, I don’t mean r-rated content (thought I’m sure there are some out there). The target audience for the seinen and josei genres are all adult aged and the narratives tend to vary based on which audience the work is targeting.
Despite targeting an older audience compared to its younger, shounen counterpart, several seinen shows could be easily mistaken for a series geared towards little girls. Much like how My Little Pony became a hit with older males in the west, the same can be said for the magical girl genre. In fact majority of recent, popular magical girl anime cater to the older male demographic. Shows very clearly aimed at otaku are also more likely to fall under this genre. Why? Because adult males are more likely to have money to spend on merchandise compared to teenage boys. While there have been seinen shows in the past that featured an adult cast, for the most part, works within this demographic tend to be littered with colorful magical girls, cute girls with one note personalities that correspond to a specific fetish, and hapless male protagonists that somehow always manages to get several attractive girls to fall for him. While they aren’t as common as the otaku-centric shows mentioned, there have also been cases where seinen works feature a narrative and cast that’s capable of appealing to a broader audience and feature a tone and story line that might as well place it from a completely different demographic…and it probably should.
In stark comparison to its male counterpart, majority of josei shows are easily identifiable as they tend to have adult aged protagonists and feature more mature narratives that tend to delve deeper into serious issues compared to its younger, shoujo counterpart. While there certainly have been teenaged protagonists and high school settings in the past, for the most part, works within the josei genre have consistently produced stories that were clearly written with an adult female audience in mind.
So unlike shoujo and shounen, there is no line between josei and seinen and it’s much easier to tell the two genres apart.