This limit is based around the content of the game and the suggestions of the ESRB and other game rating organizations across the world. It depends on your country because different parts of the globe have different standards.
You can see the listing of our sign up page: http://www.sindome.org/signup/?entry=box.
We enforce this age limit as best we can given the information we have.
This is a roleplaying game with death and murder and sex. You can opt out of the sex part, but death and murder may come to find you even if you aren't looking for it.