It will depend on the individual child to a great extent, there's probably no 'right' answer. I initially tried my (then) seven year old but he complained of sore fingers and not being able to push hard enough on the strings. He also lacked the attention span so we gave it up for a while. A guitar teacher I know agreed with me that the fingers weren't well enough developed. Now he's ten years old and we've started again. BIG difference, not only does he have the strength in his fingers, he knows what he wants to play, he's more able to do it and therefore his motivation and learning ability has increased. He's now picking it really quickly. Two important bits of advice when starting children at that age - make if fun otherwise they'll lose interest. Also, make sure they're on nylon strings -on a suitably sized guitar -A 3/4 is ideal for a ten year old.

CK