When they've shown they can use a knife properly under supervision and when they've shown they can demonstrate the proper respect for it. Being he's your son you probably know the answer better than anyone here as to whether or not he is ready.

I guess when I was young my first introduction to knives started with: "Dad/Grandpa can you cut/open this?"

Then went to, "Dad/Grandpa can I borrow your knife to cut/open something?"

Then finally to, "Dad, can I have my own knife?"

To which, by that point, Dad was so tired of me borrowing his knife he agreed to get me my own at the hardware store....as long as I didn't tell mom.