Welcome Dodge! You'll do yourself a service if you search around a bit and study up--there's some great references here. I think the OC cards might be a sticky link.
First of all, you CAN legally buy a handgun--just not from a federal firearms dealer. If you are stopped by a LEO, first ask if you are being detained. This is a yes or no question! If no, walk away, if yes--they need reasonable articulable suspicion that a crime has been committed or is about to be committed. If being detained, insist in knowing their RAS.
If it were me, I'd demand that they cite which KRS you were breaking as none would apply.
Also note that KY is not a stop and identify state. I would assume that they would also need RAS in order to check your ID. How would they even know you were 18 in the first place? They'll prolly just assume you were 21+ and go about their business.