“Seinen” manga/anime is “youth” content. The target audience is generally young adults- most often young adult males as opposed to shonen which is target to children. That said “Seinen” doesn’t represent ALL young adult content- it is a genre with certain aspects and traits generally associated. In fact while many equate ice of life to “kūki-kei” or similar- light hearted and shallower stories, many slice of life are quite deep and often graphic or intense. Seinen tends to deal heavily in action and language and some gore and such. More adult slice of life are often drama or melodrama and focus on relationships and aspects between them.
For those not familiar- saying that slice of life is for the immature and that Seinen is more adult is like saying that “Saw” and “The Expendables” are films for mature people but “The pursuit of Happiness” or “Roots” are children's stories.
For those not familiar- saying that slice of life is for the immature and that Seinen is more adult is like saying that “Saw” and “The Expendables” are films for mature people but “The pursuit of Happiness” or “Roots” are children's stories.