He did play a portion of his rookie year in Swedish juniors before he played with Frolunda's big club. Still, it is exceedingly rare for a 16 year old to play in that pro league. I'm still researching how many his age have played in the past, but I did see he became the second youngest player ever to score in that league.
Does the fact he didn't win rookie of the year diminish him in your eyes somehow? Pettersson had a great season putting up more than a point per game at age 19, after all. He's a deserving winner and a great prospect in his own right, picked #5 overall in last year's draft. I don't see what that has to do with Dahlin, though. If Dahlin were eligible last year, he would have gone #1 in that draft. He's the best D prospect since Potvin for a reason, after all.