Violence: Demon Slayer is pretty bloody. Most of the violence is against the demons, and the only way to kill demons is through decapitation. Most of the demons are dismembered vividly. Lots of limbs are chopped off. We see the results of a bloody murder. Violence is the sole reason some streaming services list it as R-rated, and it is the only area of much concern.

Sex and Nudity: There's barely anything worth mentioning. A guy's clothes are dissolved, though nothing is shown, and it's more of an immature gag than anything else. It's in the vein of "Hahaha! He's naked!", as opposed to anything sexual. A female demon slayer wears a very revealing outfit, though she's only in the last two episodes and plays a minor role in those episodes.

Theological Themes: There are demons, though they are all portrayed as evil, except for the ones who Spoiler: don't eat humans. The demons can only be killed by sunlight and being decapitated by special swords. Demon slayers have swordplay techniques that seem magical, though I don't know how much magic is a factor. The demons gain special powers by consuming blood. The more blood they consume the more powerful they get. They're more like vampiric monsters than actual demons.

Problematic Themes: Some minor, mild swearing. I don't remember anything worse than the d-word.

Overall: The story is nice, though there are a few aspects of Nezuko's state that don't make sense, as more about the demons come to light. I liked how, unlike many shonen protagonists, who have selfish goals (become the greatest hero, become the pirate king, ect.), Tanjiro's goals are completely selfless. The one thing he wants is for his sister to be human again.

Tanjiro is a bit perfect, though he doesn't come off as annoying. His relationship with Nezuko is one of the highlights of the show. The other leads balance out Tanjiro and Nezuko's quiet natures with their vivaciousness. Inosuke, a wild and competitive demon slayer (basically a feral gremlin) and Zenitsu, who's whiny, emotional, and desperate, are pretty much screaming for the entirety of the show. My personal favorites are Shinobu Kocho and Giyu Tomioka, two of the head demon slayers. Shinobu seems to be the most complex of the characters, as her personality is the hardest to peg. I liked Kanao too.

The show does have more flaws than the plot holes I mentioned above, one of which being the comedy. The comic relief character is pretty annoying and caused people to drop the show. (I personally didn't mind him that much, though I did want to slap him from time to time.)

The other highlights include the gorgeous art and soundtrack. Tanjiro's water breathing is so beautifully animated, it makes me happy. The OST was perfect for the atmosphere of the show, and I loved the opening and ending themes. I will say, Demon Slayer was definitely worth the watch.

I will recommend this to older teens and up, due to the sheer amount of blood. From what I've read of the manga, it's less brutal, so that would probably warrant a lower age rating.

The plot revolves around a young man, Tanjiro, whose entire family was killed except for his sister, who turned into a "demon" (oni). He sets out to become a demon slayer to take revenge on the demon who killed his family and, more importantly, find a way to turn his sister back into a human. At its heart, this is the story of someone who is willing to go through any amount of hardship and sacrifice for the sake of the person who's important to him, his sister. And though from the summary it might sound like this anime is about a journey of vengeance, the main character is possessed of an enormous amount of forgiveness and compassion. This extends even to the demons whom he kills: demons were once humans, and though when they become demons they forget their humanity and forget about the people who were important to them when they were human, in their dying moments, they recall those things, and usually are carrying an enormous burden of sorrow and pain. So, despite the large amount of violence in this anime, overall I find that the themes are very positive (having compassion for anyone who regrets what they did; forgiving and not holding grudges; doing whatever it takes to help your family). Finally, the storytelling, animation, and music are all excellent; I really cannot recommend this anime highly enough.

Specifics about content ratings:

Violence: 7

Toss-up between 6 and 7 on this one. This anime revolves around things like demons which eat humans to get stronger and deadly fights between demons and demon slayers. Overall, there's quite a bit of blood and violence. That said, I found that frequently, the show portrayed the violence that happens in an understated way (e.g. showing silhouettes instead of the full scene, showing the scene or a portion of the scene after a murder occurs rather than showing the murder as it is happening, etc.) However, there are a few times in the show where the violence is shown in an extremely gruesome way, and this always happens revolving around the main villain of the series, emphasizing his cruelty.

Nudity: 1

There is a prominent male character who habitually goes shirtless. There's also one female character who shows a large amount of cleavage, which is deliberately emphasized (the "element" she represents is erotic love), but in the first season she is a minor character who only appears briefly in a few episodes. Finally, there is one male character in one scene whose clothes are dissolved and who is naked, but this is played for humor and only his upper body is shown, briefly.

Theological themes: 3

I'm going to spend a bit of time here giving a detailed explanation of some themes that might seem theologically troubling.

"Demons": The "demons" in this series are actually "oni," a Japanese mythological creature more like an ogre. The interpretation of them is rather dark in this show, so I can understand why they chose to translate it as "demon," but they have absolutely nothing to do with the demons and angels of Christianity (there's a different Japanese word for that, "akuma," which is not what these are).

Sword techniques used by the demon slayers: I confirmed my impression with a Japanese friend, and basically, these should not be considered magic. It's more like the characters learn to bring out the utmost potential of the human body through meditation and breathing exercises. The elements that are shown (water, lightning, etc.) are more there as a symbolic thing to give you an idea of what the attack is like: an attack that's fast like lightning, flows like water, etc.

Magic used by demons: This, on the other hand, is basically "magic." Although it's used by demons, it's not always used for an evil purpose (there are a few demons in the series who aren't evil), but the fact that it uses blood may be disturbing to some viewers.

The dead: The souls of the dead appear and speak to living characters at several times in the series, and the main character also routinely says a prayer for the soul of the demon he has just killed. While not problematic for Catholics/Orthodox, this might be troubling to some denominations of Protestants depending on your views, so it's something to keep in mind if it conflicts with your views about the soul/afterlife.

Religion: Eastern religions are portrayed in this anime. The simple funeral rites given by the characters are basically Buddhist. There's some brief but significant portrayal of a Shinto religious dance (kagura). There is also at least one humorous reference to the "spirit (god) of the land," which is a Shinto animistic belief.

Other potential problematic themes: 2

As I've said above, I find the themes and messages of this anime overall to be positive and inspiring, and Tanjiro an exemplary character whose example we should all follow. However, there are evil characters and dark themes present, as is generally the case when there's a "good versus evil" battle being portrayed. In particular, the main villain lives up to his given name ("merciless"), and other demons, having lost their humanity, express various cruel and callous points of view and a lack of regard for human life.