First of all, let me start by saying this movie is absolutely fantastic, however, it is not appropriate for all audiences. There are several violent scenes, as well as language and some sexual content.
Violence-7/10: As for violence, this movie is generally not extremely violent, certainly less gory than some of today's movies, but there is a near constant air of suspense and there are several scenes that will make you jump. This is not to say that this movie does not have it's share of violent scenes. The film's most gory scene occurs when the characters sit down to dinner and a baby alien bursts out of a characters chest. Other characters are bitten, stabbed, and clubbed by the alien. In addition to the creature violence, there is a seen where a man's head is nearly bashed off by a fire extinguisher, although (spoilers!) he is revealed to be an android.
Sex-3/10: In one scene, there are pictures of naked women posted to a wall in the background with breasts shown. They are not easily noticeable and a bit blurry, but they are somewhat explicit. In another scene, a man makes a joking reference to oral sex aimed at one of the women. This may go over some younger kids' heads. Some of the creature and set designs have sexual undertones. This also may go over younger kids' heads.
Language-5/10: Six or seven uses of f***, several uses of b****, h*ll, s***, d***, and a**, as well as J**** and G**d***.
Drinking, drugs, and smoking-1/10: A character smokes in several scenes early on in the movie.
Now, for the review of the movie itself. This is undoubtedly Ridley Scott's finest work. The slow buildup to the fateful moment in which the alien is born is masterfully executed, leaving us with just enough detail to keep us on the edge of our seats hanging on every word. The brilliant set designs by H.R. Giger are almost as fascinating as the movie itself, with no intricate details spared. The creature design, also by Giger, is fantastic and the hydraulic head movements by Carlo Rambaldi are still convincing, even after 36 years. True, the special effects fall short at times, but that is just the movie's age showing, and there are some minor inconsistencies, but that would just be nit-picking. Overall, this movie, as well as it's sequel, Aliens, is an absolute masterpiece and an instant classic that all mature audiences ages 12 or over should watch.