I love this TV show. It's funny, quirky and let's face it.... the cast is attractive. I started watching this when I was 12, and I was fine. It may be frightening for kids under, say, 12 or 13, and under 16s who are especially sensitive, but it is actually a great show.
There is some swearing, but nothing over b**** or d***, and that is used a lot in each episode. There is also a few sex scenes, some mild talk of it, and some referencing.
The violence is the thing that pushes the rating up (the most.) There is a lot of blood and gore and violence. Not for sensitive teenagers or people under 12 or 13. Some scenes are in fact VERY frightening, with violence to rival that of The Walking Dead. Some icky and cringe-worthy moments, some that will test your queasy stomach, and there HAVE been some scenes that have made me want to cry.
But not only the violence, sometimes its the thoughts and ideas that leave you with nightmares, afraid to look out your window at night. Not so much the violence, as the implications of what happens, though violence is still a very big factor. Clowns, demons, ghosts.... Not for the faint-hearted.
I say for 12 or 13 year olds who are mature and can stomach it, but be cautious with easily-sickened children and sensitive teenagers. This could affect them. Better off for young adults in all honesty, but not completely ruled out for tweens and young teens, as long as they are mature and have a strong stomach.
If people are under 15 or 14, do not allow them to watch seasons 6, 7, and 8 if they get scared in earlier seasons, because 6, 7 and 8 are the most violent seasons and definitely the most scary. So, if sensitive and prone to nightmares, do not allow them to watch these seasons.
Ages 12 + up if they are strong and mature.