Shonen vs. Seinen: Meanings of the Japanese Words
As I wrote in this post on how the Japanese words for adults, Seijin (成人) and Otona (大人), differ in meaning,
I will talk here about the difference between Shonen (少年) and Seinen (青年) and further how Shonen Manga (少年漫画) differs from Seinen Manga (青年漫画).
Shonen vs. Seinen
Although Shonen and Seinen are both Japanese words for young males, their definitions are indefinite and differ depending on the institution, law, or the like.
Shonen refers to young boys and male teenagers aged 7 or 8 years to 15 or 16.
Seinen refers to youths (particularly males) from late teens to 20s. But when emphasizing youthfulness, its definition even includes the 30s.
Shonen Manga vs. Seinen Manga
In terms of Shonen Manga (少年漫画) or Shonen-Shi (少年誌) and Seinen Manga (青年漫画) or Seinen-Shi (青年誌),
this article on the Japanese site Manga-Gai says that, as their names indicate,
Shonen Manga (少年漫画)
Shonen Manga refers to manga comics whose target age group is Shonen and whose content at least seven or so year-old children can understand.
Seinen Manga (青年漫画)
Seinen Manga refers to manga comics whose target readership is Seinen.