great with deep/dark themes
This show is great for making you think. As such, I would generally reccomend the show for ages 14+ With the darker themes that are explored, it's safer to air on the side of caution.
Themes of sexuality are explored, but in my opinion it's not graphic enough to warrant an extreme rating. As long as the teen/child (use whatever adjective you want) understands sex and sexuality, they'll be fine. I am assuming that if they're 14 and up they've taken a health class or at least had "the talk".
Drug and alcohol usage becomes more prevalent in later seasons and (SPOILER) a character does die from a drug overdose. Once again, as long as the -insert adjective for young person here- understands the implications and consequences (them being good or bad) it should be fine for them to watch. In a later season a main character struggles with drug addiction.
There is definitely a strong theme of morality, and often the lines of "good" and "bad" are blurred. However, the show does a good job of making it evident that even though these are the choices the characters are making it doesn't make them "moral". It makes it clear that these are life and death situations, and that hard choices have to be made. It doesn't (from what I recall) condone killing, but rather considers it a necessary evil in their current doomsday society.
I won't talk much about the gore, because so many other reviews talk about it. It's there, and people who are squeamish (no matter what age) should exercise caution.
The theme of suicide becomes very prevalent in the later seasons, so I do think people should be aware of that. I'm not overly fond of how the show deals with it, but we can't have everything. Its seen mostly as a way out, and it's kind of hit-or-miss when it comes to characters seeking/getting proper help or support.
Character deaths do happen regularly, just a warning
Over all, I would recommend this to older teens/adults who are willing to think about the messages this show presents. It definitely isn't a mindless watch. .