While a particular LEO in CT might apply the law that way, Navy LT, it doesn't seem to me that someone 18-20, legally in possession of a handgun, would have it forfeited. If that person made the handgun, or transferred it in from out of state, there would be no violation of the statutes, so the "in violation of any provision of this section" clause wouldn't be activated. Proving it could be a little difficult, and I'd expect the handgun to be confiscated while the young person is proving they acquired it legally, at least in most CT towns and with the attitudes I've heard from most CT LEO's, but there's nothing in the law which calls for that.
I suspect such a handgun found in the possession of such a young person in any place other than that young person's residence or place of business would be confiscated, as the person would be in violation of the State's laws about carrying and transporting handguns, which is illegal without a permit and permits only available to those over the age of 21, but that's no different from someone over the age of 21.