I am kinda convinced that Yami actually aged a little despite being dead and sealed in a magic box for a few millenniums. He looks and sounds like he's 13 in the season 0 and then the first season on he looked and sounded a few years older. Like maybe 20-ish????
But his behavior seemed to change when meeting Kaiba and Mokuba I think. Not sure.
You ask when Yami became the nice Yami we all know? It started happening somewhere along the way in the manga, I can't recall..maybe a few volumes into the manga. But yeah...at the very begining of the manga/series 0... he's nearly as scary as Yami Malik. :'D *hides*