My son received his first pocket knife at 5, it was an old Barlow whose edge was worn a bit -- I I took a stone to it and wore it down a bit more. Over the past few years he has slowly advanced in knife quality as he has learned to use it. About 6 months ago, he bought a Buck Vantage (with his own money). He knows the knife is a tool, not a toy. In fact, over the past year, him having a good knife as made my life a bit easier -- I don't have to get off the tractor to cut hay twine off of round bales, at our bi-monthly stock shows I simply throw the shavings bags where I want them and he makes the beds, etc..
All that being said, he's a typical 8-9 year old boy that makes a lot of bone-headed decisions (a year ago he "gifted" an old pocket knife of his to a friend at school - that was fun!); but if you don't give them a chance to have responsibility and a chance to fail, then they may not get to where you want them to go.