This is actually a really good question. In "the Initiative", in the famous "can't bite" scene with Willow, he says "I'm only 126!" But if he is counting in vampire years, he's actually 119. (1999-1880=119) So....if he's counting the years that he was human...then, well, he had to have been turned when he was seven. Just another one of those continuity problems, I think!