We'd generally advise 16+. Many themes of horror do trend towards an adult rating
However it IS worth noting that while there's a little bit of blood in some images and some slightly nasty descriptions of things there isn't really anything extreme or adult in the books.
Additionally the focus of the horror is put in the players hands as it primarily comes from the characters chosen Haunts (which can be whatever) so it does actually scale to the players' interests/level of horror to some extent.
In terms of complexity we think the game is quite approachable but does ask for a level of engagement on the themes and concepts of the horror. What we've previously said is that Locus doesn't really need experience with rpgs but will benefit from experience with horror in some form (though kids horror would still be entirely appropriate for this).
tldr, you can do a kid friendly locus game but it requires a little more care around the themes and content of the story than an adult game might.
Thanks for asking!