It depends where you are really. In the UK, it is illegal to sell most knives, including kitchen knives, to anyone under the age of 18. However, in Scotland, the law is slightly different—16 and 17-year-olds can legally buy cutlery and kitchen knives.
Retailers are expected to verify age before selling knives, and they can refuse a sale if they cannot confirm the sellers age (which you did) or suspect the knife might be used unlawfully and I would say that buying one knife (depending on the type) could give rise to suspicion. Did you ask him why he wanted just one knife?
Having said that, I have purchased just one Sabatier knife as my knife set was blunt as hell.
If you're a retailer and unsure about a sale, it's always best to err on the side of caution. which you did. But if I was your boss, I would ask why you hadn't referred it to me before declining the sale, if you had concerns? I think that is the question you need to be prepared to answer.
Retailers can refuse to sell a knife to an adult if they have reasonable concerns about its intended use (you have the benefit of knowing what he looks like, I do not) . While there is no specific law requiring retailers to deny sales based on suspicion, businesses are encouraged to act responsibly and follow due diligence when selling bladed items.
If a retailer believes the knife may be used unlawfully or poses a risk, they can refuse the sale and report concerns to the authorities. Many stores also have store policies that allow staff to decline sales if they feel uncomfortable with a transaction. I'd be looking at the stores policies if I were you BEFORE you see the boss and see if there is anything in there to support what you did