How old should kids be before playing fatal frame

The Fatal Frame games are rated M (17+ or 15+ depending what country you live in) for horror themes.
but I would say that 13/14 year olds could play it fine (unless they don't react well to horror movies). If your kids are used to watching horror movies, then they would have no problems with the Fatal Frame games.
The Fatal Frame games are pretty tame compared to most horror games. There is no gore, no uneccessary violence. Most of the scares are centered around creepy atmosphere and dark/claustrophobic rooms. They are very similar to the Japanese horrors that have been popular recently.
If you are unsure about anything, here is a helpful link for people who are unfamiliar with the Fatal Frame series. It details what the games are about and has a trailer/video to show you what type of scares the game has:
http://www.neoseeker.com/forums/38474/t1104690-if-you-not-familiar-with-fatal-frame-project-zero-series/
NAOL