Technically Ba'al does age. He just has the Sarcophagus to prevent death, and even it wears out eventually. Before Lord Yu got shanked the guy had gone pretty much senile as is, so that would theoretically happen to Ba'al as well. Just not in the time scale we care about. And without the Sarcophagus tech he's a lot more vulnerable than in regular SG.

Also, Ba'al has a persona that doesn't scream 'evil', and he's gone to great pains to hide the fact that he's a manipulative, megalomaniacal, egotistical bastard.