There is no exemption to the age limit for buying knives, other than to buy razor blades with less than 1mm of exposed blades (think gilette razors) or where the knives are designed for domestic purchases (age limit is 16 here), however many stores make it policy not to sell anything sharp or pointy (or blunt in some cases) to under 18s.
I believe the law does somewhere state that, like with alcohol and cigarettes, an over-18 can't buy a knife on behalf of an under-18, however, if it is gifted, common sense prevails in the majority of cases.
I won't go into the legislation as it's covered in various places on this site and the Internet, and I'll no doubt bore everyone to sleep, but if and when you get the knife, however that may be, just make sure you know, understand and act upon what the law says. I agree, some of the laws could be changed and possibly improved, but knowing what they state will keep you on the right side of the law. All it takes is for an action to be taken the wrong way or someone to call the police saying a "youth" is walking around with a knife and you'll end up having to explain why you've got it. I know it's a pain but until the law is changed, I wouldn't imagine it getting any better.
That said, enjoy the knife!