Ha ha, that's a hard question to answer as it's completely down to the individual and the adult with them, Thorin our son had a knife at 5, the girls had them young as well and used them quite regularly under supervision, he prepped fish, fires etc etc from that age and before, but he was contentious and I was confident in him and my ability to keep an eye on him and help. Some kids are considerate and sensible, some are not so it's a judgment call. As for a knife, I just made sure that they were small enough for a good grip in their smaller hands, i've got knives wit blunted tips that we use sometimes, I especially use those with kids I'm notes sure of or don't know.
To a point you just need to get now it it (within reason) and make the call on how happy you are. Obviously have a first aid kit and know hot to use it, get the safety stuff covered and the kid well indoctrinated on safety, their own and others and then get on....