However, it appears, according to GA code, that anyone within the age of 18-20 is not prohibited from possessing and owning a pistol. But, they would be prohibited from carrying it in their motor vehicle ( since they are ineligable for a license due to age ) unless unloaded and encased and prohibited from carrying on their person unless they are engaged in legal sport shooting and/or hunting and fishing, or within their own home or place of business.
As long as it is a private transfer and you sell it to him knowingly that he is not otherwise prohibited from possessing a firearm ( Felony charge, illegal alien, ect) Then I don't see where state law is violated. You still need to check federal laws though. Here are some helpful links.
From whom may an unlicensed person acquire a firearm under the GCA?
A person may only acquire a firearm within the personâ€™s own State, except that he or she may purchase or otherwise acquire a rifle or shotgun, in person, at a licensee's premises in any State, provided the sale complies with State laws applicable in the State of sale and the State where the purchaser resides. A person may borrow or rent a firearm in any State for temporary use for lawful sporting purposes.
[18 U.S.C. 922(a)(3) and (5), 922(b)(3), 27 CFR 478.29 and 478.30]
So in my non-legal opinion,
I would say YES it appears that it is legal for you to sell this 19 year old your XD pistol, provided that you sold it to him knowingly that he was not otherwise prohibited. And that you're not a FFL licensee and you both reside in the same state.