I've been a fan of the Metroid Prime games ever since I played the first one at the age of 7. After playing this one, I found that it is probably the most dark of the series.
Like any other Metroid game or any other first-person shooter, there is LOTS of violence. All the enemies you fight are strange alien creatures, and some of them may disturb younger children. There is a lot of blood but it is very animated and not realistic in the slightest. Though upon dying in the game, we see a growing pool of blood, assumed to be that of the character you play, but it is not very graphic. During the whole game your character is being corrupted by a radioactive substance, and we frequently see cut scenes of the substance growing larger inside the character's body, but then again, it is not graphic or detailed, but the idea is dark and slightly disturbing. The story overall is dark, and a little bit sad (I won't explain the sad bits due to spoilers.). I would recommend it for 11 year-olds who aren't particularly disturbed by alien creatures, unrealistic blood splatter, and dark themes. Other than that, it's a great game.