Seconding Berserk! It's recently had a bunch of fancy re-releases and they are not at all easy to tell the content from the cover (since the covers are more stylised, you can find them on Amazon yourself and see what I mean). However the manga is GORY, is filled with many many intense themes (even for adults it can be a hard read!!), and plenty of teenagers do end up reading it because of like Guts (main character who goes around with a larger than life sword).
I'm assuming the 13 year old is probably more naive than the average 13 year old (or that the adult in question is particularly strict about age restrictions), in the sense that many 13 yos these days have been exposed to so much stuff through the internet that something like Berserk isn't likely to bother them in the same way that it might have to 13 yos in the 80s/90s (and plenty of them read Berserk then too).
But yeah, Berserk is a 100% realistic thing someone might pick up for a 13 yo that another more knowledgeable adult might go 'ahhhhhhh no this is not My Hero Academia this is a bit much.'
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I'm assuming the 13 year old is probably more naive than the average 13 year old (or that the adult in question is particularly strict about age restrictions), in the sense that many 13 yos these days have been exposed to so much stuff through the internet that something like Berserk isn't likely to bother them in the same way that it might have to 13 yos in the 80s/90s (and plenty of them read Berserk then too).
But yeah, Berserk is a 100% realistic thing someone might pick up for a 13 yo that another more knowledgeable adult might go 'ahhhhhhh no this is not My Hero Academia this is a bit much.'