These days it's actually pretty rare for a movie to receive an 18 rating in Europe. Most movies containing moderate amounts of gore/sex/profanity etc tend to be given the much more lenient (and lucrative) 16 rating. So when a movie does end up being certified 18 it tends to be fairly hardcore piece of cinema one way or the other. As you can see below, Spanish theater chain Cinefilo have assigned Prometheus an 18 certificate along with their synopsis..
Now it's a safe bet that the UK and Ireland at least will follow suit, and quite probably the US too, meaning the movie will be rated R there. Of course, it's still a couple of months away from release so cuts may be demanded along the way in certain Countries to secure a more "box office friendly" rating, but if everything remains the same it seems Scott will be going old school and delivering a real chiller.
Prometheus is directed by Ridley Scott, from a screenplay by Damon Lindelof and Jon Spaihts. The film stars Noomi Rapace, Michael Fassbender, Charlize Theron, Guy Pearce, Idris Elba, Sean Harris, Rafe Spall, Logan Marshall-Green, Patrick Wilson and Kate Dickie, and is due for release on June 8th 2012 in the USA, and June 1st 2012 in the UK.