Short answers, pulled from the story.
The Japanese word shōnen originally meant young boy in a general sense. Publishers used this term until the end of the 19th century to label publications for children and youth before shifting definitions during the early 20th century.
Publishers began openly adopting the categorization system starting in the 1960s. By that time, the term specifically designated media aimed at adolescent boys rather than all young people.
Weekly Shōnen Jump reached peak circulation of 2,380,000 copies in 2015. This figure far outpaced competitors like Weekly Shōnen Magazine which sold 1,110,000 copies.
Osamu Tezuka pioneered story manga with cinematic continuity across chapters. He created works such as Astro Boy and Kimba the White Lion under strict occupation guidelines after World War II.
Comics theorist Neil Cohn describes shōnen art style as generally edgier than shojo manga. Protagonists often possess insanely spiky hair and significantly smaller eyes than those found in shojo manga.