Short answers, pulled from the story.
Seinen manga is an editorial category of Japanese comics marketed toward young adult men, typically those aged 18 to 30, though some definitions extend the target demographic up to age 40. It is one of the four primary demographic categories of manga, alongside shōnen, shōjo, and josei.
Seinen manga emerged as a recognized category in the late 1960s. Comic Magazine, launched by Hōbunsha in May 1966, is identified by scholars including Yoshihiro Yonezawa as the starting point of seinen manga.
The Publishing Science Research Institute distinguishes seinen magazines, which carry mainstream adult titles from major publishers like Shueisha or Kodansha, from "mature" magazines that contain pornographic material produced by smaller specialist presses. Major publishers use the term seinen, which literally means "youth," to avoid stigma around adult readership.
The New Wave movement of the late 1970s and early 1980s introduced experimental storytelling and a cinematic, realistic visual style to seinen manga. Artists like Katsuhiro Otomo brought philosophical approaches to science fiction through magazines such as Young Magazine and Big Comic Spirits, and female artists including Rumiko Takahashi began contributing emotionally complex narratives to the category.
By the 1990s, seinen manga made up around one-third of all manga output in Japan. The category's growth in the 1980s was described as one of the main drivers of the overall expansion of the manga industry during that decade.
Seinen manga makes heavy use of fast-changing perspectives, varied panel compositions, speed lines, subjective motion, and onomatopoeia. Scholar Thomas Lamarre has described its mode of address as oriented around the role of the observer, with readers frequently positioned as third-party viewers of on-page events.