Yeah it's kinda the same as with the Ted deal, Ted looks way younger in the original Suikoden then he does in in Suikoden IV somehow. I think that it is just a case of the artist's not researching/caring what the older pictures look like or using them as a reference.
That isn't necessarily true. If you check the art for Ted, there are differences just between the pics for the first game. But besides that, I think Ted looks the same age, if not younger, in Suikoden IV... except he's miserable. Which is no surprise -- think about how stressed he was back then. And his facial expression reflects that level of stress, which might give the impression that he looks older. But when you check out his art for the first game, he isn't exactly younger looking...he's just smiling more. And again, it's just a reflection of how his life is at the time. He's got good friends, Windy isn't trying to take his rune (not right then), he's a chatty Cathy... he's happy as a clam.