I adore this TV show. It's hilarious, emotional, relatable, and once it gets ahold of you, you can't escape. :P I began watching it about two months ago at age 17 and am just starting season 8. One of the best things about the show is the emphasis on the importance of family relationships. I love Sam and Dean's relationship and the distance they're willing to go for each other. In almost any story, it seems like the 'main relationship focus' is always a romantic one, and I like that this show focuses on relationships other than just romantic ones. It shows that other kinds of relationships ARE important. I also like the message it sends of doing what needs to be done for the benefit of others. Sam and Dean, in many respects, are wonderful role models. They're brave, self-sacrificing, and know that they have to do whatever they need to do in order to protect anybody--whether they have a personal connection to them or not.
Now, that being said, I consider this show a 'filtering through for goodness' kind of show. Yes, it has its muddy spots that I, personally, think the show would be better without. There is sex, drinking, language, and violence--I'm not going to pretend there's not. As far as the drinking and violence goes, it's not too big of a deal. Sam and Dean are over 21, and though I have high standards for myself to not drink, their drinking doesn't really bother me. They're men, they like their beer, and they usually don't get drunk. Sam does sometimes (probably because he has a lower tolerance for liquor than master-drinker Dean), but those scenes are mostly just humorous. The violence is to be expected for this kind of show. It's not particularly realistic, and usually not over the top, but there are some pretty graphic parts, such as a man's arm getting ripped up in a sink's disposal or a guy's head getting chopped off by a garage door... I guess that is pretty bad, but for the most part the violence isn't anything more than shooting a ghost with salt or Sam or Dean getting shoved across the room and running into stuff. The thing with the violence is that the show doesn't promote violent behavior; the violence occurs as a result of the bad supernatural elements and Sam and Dean have to stop the violence. The language/dialogue is definitely something you probably wouldn't want a kid hearing. Swearing is common, though no S or F words, and there are some themes discussed that are a little iffy.
My biggest problem with the show (but I love it so much I just put up with it) is the immoral behavior promoted through the show. I wish there would just be an episode where Dean finally makes a decision to grow up and not sleep around anymore. That would make me happy. See, part of Dean's character that is generally easy to kind of ignore when he's acting nobly is that he's a sex guy. He loves women, he loves looking at them in ways he shouldn't, and totally sees them as objects (mostly). Sam isn't quite so bad, but he does hook up a few times during the series, which always seemed kinda out of character for me. This is a theme in the show that is not good, the theme that having sex with anyone you want is fine, perfectly acceptable, that these cool guys are doing it and nothing ever happened to them. Now, there is nothing pornographic in any of these scenes, just intense make out sessions and carefully placed camera angles so that you can't see anything. And out of 200 episodes, there are probably only like 6 or so of these scenes. Anyway, this theme right here is why I would suggest this show for being for older kids. If I would've watched it when I were 11 or something, I would've thought it was a terrible show. But, because the rest of it is so amazing, this part is just something you have to put up with to get to the goodness.
Now, beyond that, there is something AMAZING that I would like to address about the Supernatural experience, something that gives it probably the most positive effect out of any show, and that is the actors behind the characters. Each of the actors behind the main characters, Sam, Dean, and Castiel, are loyally married and have children whom they love. They don't ever do anything stupid like so many other actors do, they are generous and kind, and they even say that they don't really want fame and that their families are the most important things to them. I have never come across any other actors who are such good role models to their fans as these men. They have their heads on straight and seem to really know what matters, and I think that seeing that is one of the most positive things that can come from watching Supernatural.