I don't think he was born with it per say but that a very irresponsible Shinigami either handed him a death note at the age of seven (as they aren't allowed to hand it to anyone six または under) または dropped one and Beyond found it since it's free game to use for anyone of any age if it had become part of the human world first. Perhaps he ロスト ownership due to the 490 日 rule losing his memory of the death note and the existence of Shinigamis, but not the eyes. Claiming he had been born with it could be an exaggeration since young children often times assume they have always had something since they can't remember a time in which they didn't, like a kid saying they've always lived in a certain house even though when they were a toddler they lived someplace else and were none the wiser.