10+ if they're mature, 13+ if they're new to some... topics.
Great show, more of a drama than anything, watched a bit of it when it first came out and I was about 6 years old, of course my eyes had to be covered at times, but ignoring the sexual scenes which at the time I couldn't watch (They still are a little uncomfortable for me lol) it was a really good show to watch which helped me understand specific dangers and safety. I've been keeping up with the show regularly for a good 5 years now, and it never becomes too graphic or violent. Never have I had to stop watching because of some graphic content. I think it is because it is in a realistic context, but it is not too gruesome, unlike 911.
There's no necessarily BAD characters that are constant, but when they appear they're dealt with appropriately which again, is a + for positive role models because it shows that people can be dealt with without violence.
All of that aside, great to watch with friends, family if you're a teen, and never has been boring.