United States of Tara is Rated Mature, but that is only because it has a lot of intense language (e.g., f*ck, c*nt), which actually makes it more realistic if you think about it. Other than that, it's nothing out of the ordinary. It does have some sexual content, but that is only like the PG-13 type. In Season 1 there was like two very brief scenes of nudity (one being breasts (but not in a sexual way) and the other being a butt (a man masturbating), but it is nothing graphic. If your teens are mature enough, they can definitely watch the show.
It's actually a great "dramedy," that at at least one point of the show, you can all relate to. I think that why it canceled was because it got a bit darker with less comedy towards the end. I mean United States of Tara was always a very *serious* comedy-drama, but the comedy from earlier episodes seemed to fade away and become a bit dark, but it was still a very excellent and great show that should be continued, but unfortunately that might not happen.
It's on Netflix and on Hulu Plus so if you like serious dramedies then watch it. :)