That can be a very personal decision based on the maturity of your boy. I know in Cub Scouts, they do not officially earn the cub version of the whittlin chit until Webelos. That is around 9 to 10 for most. Based on my experience, roughly 3/4 will not have had their own knife before Webelos. Most all will have their own knife by the time they graduate from cubs in the 5th grade or 11 years old. My son got his first about the time for crossover from Bear to Webelos.