I'm assuming you're American but the BBFC provides some very good advice on the content of films. They gave Alien an X classification what it was first released, meaning that nobody under 18 could see it, and subsequently gave the director's cut a 15 classification in 2013. Here's what they have to say about Alien:

The film contains five uses of strong language ('f**k'), as well as some milder bad language.

There are scenes of moderate violence and horror as the crew are chased and attacked by the alien and as they attempt to fight it off. In general, the violence occurs offscreen or in semi-darkness, although there is some sight of blood in the aftermath of alien attacks and sight of a character being bitten. Scenes of horror include sight of an alien creature attaching itself to a man's face and subsequent sight of the alien bursting out of his stomach, with some gory detail. There is also sight of a robot character being decapitated and spewing white fluid in place of blood.

ALIEN also includes sight of adult character smoking cigarettes.

Generally we're less tolerant of violent content than Americans (in turn, Americans are less tolerant of sexual content than we are). So you might want to take that into account.

Personally speaking I'd probably be okay with showing Alien to an 11/12 year old. It might shit him up terribly, but then, kids should be terrified every now and again. I have great memories of catching things late at night on TV that I didn't understand and scared the crap out of me. Now that I'm older I know what it was that I saw, and my perspective on it is different, but it was still important that I was confused and freaked out at a young age. Getting a glimpse of the adult world that totally confuses you is one of the joys of being a kid.