Wonderful film, might upset younger children
First of all, Coco is a great movie. It's an entertaining adventure with emphasis on the importance of family. As for age rating, it really depends on the kid. Some children as young as four might enjoy it, even if they don't fully understand the plot, but there are some scenes that might scare or upset young kids.
SKELETONS: You can probably tell from the cover that a majority of the main characters are skeletons. They're presented in a very non-creepy way, with colorful clothing, facial features, and the understanding that they're deceased relatives. Still, I know some kids are scared of skeletons, and there are a few antics with tossing around bones that could freak kids out. Overall, though, if your kid doesn't have a problem with skeletons, they shouldn't be a big deal.
DEATH: A major theme of the story is death, since it centers around the afterlife. In the Mexican tradition, deceased relatives can cross over to the land of the living for one night if their families put up their photos to honor them. But if nobody remembers them, the spirits fade away to an unknown final death. We see one minor character fade away, and a major character almost does. Also, a living character's death is implied.
One thing some people seem to be upset about is that one plot point is that a character was murdered. It's not shown in a violent or even slightly gory way; he is poisoned and collapses. Most kids understand the concept of killing and death if they've read or watched any piece of media, so I don't understand parents getting upset about this. It's portrayed as a wrong thing to do.
GREAT MESSAGES: The movie has some really good lessons for families. The main character does disobey his parents in order to follow his passion for music, but everyone ultimately learns to put family first-- which sometimes includes making mistakes and apologizing. The main character discovers that pursuing your talents and dreams is important, but not to let ambition lead you away from people you love. Coco is an amazing film to watch with your family.