It is a subjective question. Watch how he cares for his other personal items. If he abuses them, if he doesn't take care of his stuff now, then a knife would be inappropriate I would think. If he treats other toys like play weapons, then he's probably going to want to do the same with a knife. I got my first pocket knife at 9. Grandad gave it to me, and he showed me how to care for it, what I could use it for, and told me what I couldn't do with it. I did what he said, and I kept it till mom took it away from me as punishment for some other transgression, then promptly lost it (I suspect purposefully).

My point is, you don't just give a kid a knife, anymore than you would give them a gun, or a power tool, or your car keys. You will take the time, as Grandad did with me, to demonstrate to them the proper use, care, and respect it deserves, and reinforce those values often, until there is little question the kid understands and accepts the responsibility. It is not so much dependent on the kid as it will be on you, for if the kid abuses it, that mistake will rest on your shoulders, not his. If done the way it is supposed to be done, then it's possible a 6 year old could have a pocket knife. Recall that many a pre-teen boy would go out with a 33 caliber squirrel gun and fetch up dinner as little as 50 years ago in some places. They were taught respect along with how to use and care for the tool, and so it was never abused.

You make the call when you feel you are ready for the responsibility, not when you think the kid is. That's what I did with mine, and never had a problem.