Hello,
I think it depends on the child, my oldest got his first knife (the opinel one with the rounded nose) when he was 4 years old but it was "issued" to him to use when we were in the woods and then put back into Daddy's "stores", that way he only had it under my direct supervision. For his 5th birthday/Christmas (they are very close together) he got the Bear Grylls penknife, fire steel and head torch, with the same rules applied for issuing it in and out. He's very sensible with his knife and see's it as a tool and not a weapon like he does his legoland toy swords etc.
My youngest is not 4 until march but his different to his brother and I don't think he'll be ready to use one before he's 5 or even 6 maybe as he just isn't as responsible.
I have seen kids as young as 3 using large machetes to open coconuts etc while I was out in Siera Leon with the Army and they were managing not to hurt themselves.
As I say I think it depends on the child and only the parent will know when they are ready.
I gave my oldest an old, fairly blunt and snapped hacksaw blade with black insulation tape for a handle to "play with" in the garden when he was about 3 years old or just before. This worked well as he got used to using it but couldn't really do himself any damage unless he sawed and sawed at himself and frankly he's not that daft lol
Hope that helps
Cheers,
Bam.