There are plenty of reasons that Alien might be deemed a bit intense for younger viewers. After all, that scene where the little xenomorph tyke bursts out of John Hurt's chest has given many a grown person nightmares. But it wasn't the violence that earned the film an X-rating from the British Board of Film Classification in 1979. No, it was because the movie offered a "perverse view of the reproductive function." Yup, it was all about sex monsters.
The Daily Mail looked into the BBFC archives and found that Alien would have been awarded a AA rating (deeming it appropriate for viewers 14 and older), but received the 18-plus X-rating because of the scene in which Hurt's character Kane inspects the pulsating eggs. In the movie's file, the censor explains what about the scene made them uncomfortable:
I feel uneasy about passing for 14-year-olds a film which uses sexual imagery in a horror context. The images are not always explicit but run like a dark undercurrent throughout suggesting a powerful, threatening, unnamed force. Occasionally the image is explicit as when the leathery egg opens up to reveal a glistening pulsating membrane which erupts into a squid-like creature.
The censor goes on to say that these feelings might be confusing to younger teens:
The early teens are a troublesome time with physical changes making terrific demands on emotional stability. I don't myself want to pass for this age-group a film which might be disturbing in a non-specific way to a significant proportion of them.
Personally, I think "disturbing in a non-specific way" is a great way to describe puberty in general.