Short answers, pulled from the story.
The first long-form gekiga appeared in March 1966 within Bessatsu Weekly Manga Times. This story by Takao Saito marked the beginning of a cultural shift that redefined Japanese storytelling for decades.
Scholar Yoshihiro Yonezawa identifies the launch of Comic Magazine by Hōbunsha in May 1966 as the true beginning of the genre. This magazine was created to target older readers and effectively established the category of seinen manga.
Shueisha launched Weekly Young Jump in 1979, Shogakukan launched Big Comic Spirits in 1980, and Kodansha launched Morning. These publications targeted salaried employees and promoted themselves as offering quality entertainment comparable to novels or films.
Seinen manga emphasizes action and uses fast-changing perspectives, varied panel compositions, speed lines, subjective motion, and onomatopoeia. The mode of address is oriented around the role of the observer, positioning the reader as a third-party viewer of events.
By the 1990s, seinen manga made up around one-third of all manga output. Attempts to launch magazines for the aging postwar generation, such as Big Gold, met with limited success and were eventually discontinued.
Major publishers use the term seinen manga to avoid official scrutiny and stigma surrounding adult readership. The Publishing Science Research Institute separates seinen magazines from mature magazines that include sexually explicit or violent works.