I have been a Police Officer in CT for nearly 22yrs. I am unaware of any statute that sets an age requirement for the purchase of knives. Individual stores can and do set their own policies on whom they will sell knives to and, while it is legitimate for them to do so, these policies are not law. The store tells you that their policy is "the law" so that you will not give them a hard time. Another example of this practice that you have probably encountered is that most stores post signs saying customers are required by law to wear shirts and shoes. The stores may choose not to serve you if you do not comply but there is no actual law mandating the shirts and shoes.
There are two main Connecticut General Statutes that deal with knives; Sec. 53-206. "Carrying of dangerous weapons prohibited"
53-206, and Sec. 29-38. "Weapons in vehicles"
29-38. There is supposedly another statute that mandates the expulsion of students who bring knives with blades over a certain length into a school. However, this would not be a criminal statute, and I have been unable to find it.
53-206 used to state that, within 24hrs of a sale, the seller of any such deadly weapon or dangerous instrument was to give written notice of such sale to the chief of police of the city, warden of the borough or first selectman of the town where such weapon or instrument was sold or delivered , with a $100 fine for every day that the seller was not in compliance. This statute was so broad that anytime K-Mart sold a set of kitchen knives, for example, they should have reporting the sale and the buyer's information. I have never heard of this statute actually being enforced, and they did away with this provision when the statute was revised in 1999.
Interestingly, when 53-206 was revised in 1999 they made an exception to the prohibition on carrying a knife with a blade 4" or over in length. The law currently says that persons with hunting or fishing licenses are exempt from this length restriction. Note that it does not say that you have to be actively hunting or fishing. I contend that the statutory exemption applies at all times but other, less enlightened, Police Officers may disagree.
I should point out that the statutes prohibit knives where the "edged portion" of the blade is four inches or over in length. Most Police Officers measure the blade by placing it next to a ruler and counting the inches between the tip and the end of the cutting edge. However, a growing number of Officers are measuring the actual cutting edge, which on a curved or recurved blade may be longer than what would normally be considered the overall length of the blade. I have to admit they are correct, but that's not how most people would think to measure a knife. I recommend checking the actual cutting edge of your knife, as you may find that that knife that you thought was just under 4 inches really has a cutting edge 4 inches or longer. Also, the statute prohibits "dirk" knives but the definition of a dirk knife is not in the statutes (I have looked.) In my search for an official definition I have found that dirk knives are generally considered to be any dagger or other knife designed primarily for stabbing. At the CT Municipal Police Academy (POSTC) they usually teach that a dirk is any double edged blade. Since the statute prohibits all dirk knives, no matter the length, you should consider any double edged blade to be illegal in CT. Of course, you can keep any edged weapon you want in your house. Just be sure to leave it there.
As liberal as CT is, I find that our general statutes dealing with knives are remarkably reasonable compared to other states. We don't have any prohibitions on concealed carry and the laws allow for knives used by sportsmen. The statutes are less concerned with the fact that you have a knife and instead concentrate on what you do with it. The Police are usually on the same wave length. If you have an illegal knife but are not doing anything else wrong they will probably just warn you to leave the knife at home. However, if you are obnoxious to the Police and/or are up to no good, you can expect to get arrested and charged with either 53-206 or 29-38. Attitude counts a great deal!
If you tell me what kind of knife you were carrying (a picture of the knife next to a ruler would help) I will give you
my opinion on whether or not the knife is "legal" in CT.