Conan says that by the time he was 15 his name was 'repeated about the campfires' to m that smack of considerably more than 'potential', more that men were recounting his deeds. And he'd gained even more experience by the time he fought with the Aesir, youth though he still was. 6th level seems to me to be a realistic estimate. Just because he's inexperienced in the ways of civilisation doesn't mean he's low level. Nothing in Frost Giant's Daughter suggests inexperience on Conan's part - because he isn't.