It's rated TV-14, which is basically MA. So this is just me but I'd advise parental guidance for anyone under that age, not just for the moments that are violent and graphic, but also for some of the dialogue, eg. episode one contains a homophobic slur and without proper context a younger audience member may not understand why it's being used. The cussing is fine, there is one f-bomb in the entire series so far.
But yeah the violence, blood, gore and even the more supernatural/psychological horror, are present from season 1 and it only increases as the seasons progress. There's also stuff like Mike jumping off a cliff, amongst a number of other examples where the young protagonists endanger themselves and experience trauma.
While I know most young adults would not seek to copy that kind of behaviour in real life, having an adult present to explain why Mike or the other characters would do such a thing and act in ways because of trauma they've experienced, eg. Will's paranoia that he's going crazy in season 2 and his destruction of Castle Byer's in season 3, is reasonable.