Rio, are you a parent?
Generally, buying a knife is a restricted item to those over the age of 18, unless accompanied by a parent/legal guardian. I, for one think this is a good thing and do not have a problem limiting/restricting knife sales to adults over 18.
I also have no problem with a minor carrying and using a knife properly, providing they will treat the privilege with the associated respect and training. However, as a parent I want to know what my kids are buying, so I can ensure they receive the proper training and understand my expectations of proper knife handling.
Pete