Age rating/recommendation for Dwarf Fortress

As for difficulty I think an interested thirteen year old should be able to learn to play. Oftentimes it seems children are better at learning interfaces than we adults are. As far as content dwarf fortress contains the following.

• Fantasy Creatures
• Cannibalism (Elves)
• Kidnapping (Goblins)
• Torture and torment of captives
• Brutal law enforcement
• Violence
  • Blood spatter
  • Overly descriptive organ damage
  • Vomit caused by stomach damage
  • Protruding organs
  • Failed surgical procedures
  • Permanent mutilation from violence (lost eyes, hands, etc)
  • Violent death of children and infants
• Madness and emotional torment
• Corpses which need to be moved
  • Graves, mass and otherwise
• Ghosts of the fallen haunting the living
• The need to clean up vomit and blood
• The ability to kill unicorns, and mermaids for their valuable body parts
  ^{You can still kill good creatures, but their body parts values have been lowered to make the practice less attractive}
• Totems made from the dead
• Alcohol and alcohol dependence
• Monsters kidnapping people and turning them into monsters for procreation

Dwarf Fortress does not contain:

• Drug references, aside from alcohol.
• Mention of sex, nor sexual topics. Creature procreate, but the mechanism is not specified.

What tempers the excessive gore of the game is the fact that it is text based. Very gruesome things happen in game, but are not visualized, only described in prose. You would have to determine from that how appropriate it is for the child in question. I'm fairly sure the ESRB would rate it at least 13+.

It's also worth nothing that the community around Dwarf Fortress can be quite gruesome and disturbing, including the official forums.