I believe you have to be 21 to buy an "assult style weapon". Every time that I have purchased an AK or AR I have had to show my permit to acquire handguns. The seller has had to make a photo copy of it to put with the transfer form. Of course, this has been at gunshows and gun stores.
A couple of months ago I called my local sheriffs department to find out the legality of selling one of my AK-47's to a cousin. I asked if he had to have a permit to acquire handguns in order to buy it from me. The first person I talked to told me no, I could sell it to whomever I wanted, and that there were no age restrictions. However, they recomended that I keep a record of the sale in case the firearm were to be used illegally so that I could protect myself from any legal issues. They also said that if I "trust" the person, that I could go ahead and sell it to them, because firearms are not (yet) registered in this state. Wanting to confirm what I had been told, I called back a week later and talked to a different person, and he told me that I could NEVER sell an AK or an AR to a private party, only to a FFL holder, because it was a "dangerous weapon". No help there.