This reminds me of certain police forces routinely stopping and searching young black men, because "Let's face it, there's a lot of crime happening and a lot of them are probably involved, aren't they?"
If a single knife is a strange thing for somebody to buy, why do shops sell single knives? Or is it not strange if a 63yo woman was wanting to buy one?
What if your own kitchen knife has just broken or gone blunt and you need a new one, but you don't have money to waste on extra unneeded items, just so that your basket as a whole meets with other people's approval? He might even have a knife set at home, but this one specific knife has broken whilst the others are still fine, so why would he need almost two complete knife sets in total, if he buys a whole new set just so strangers won't judge him?
Something tells me that, if he had also had a tin of soup, a bottle of shampoo and a tape measure, people would be accusing him of 'probably' deliberately buying other items purely to try to make the knife purchase less suspicious!
At any rate, if somebody is wanting a sharp kitchen knife for criminal purposes, why wouldn't they just avoid all of the hassle of being quizzed or suspected by the assistant by slipping it into their bag or inside coat pocket and not even attempting to pay for it in the first place?
If he's 18, for all we know, he could even be in the army and trusted with all kinds of weapons whose sole purpose is to harm and kill people; but he's prevented from buying a knife to chop his potatoes when he's at home!