The lack of specific guidance on this question, even from NHTSA, is startling. The best advice is, under 12 in the rear seats. First of all, although most of us do that when we can, the question obviously comes up when for some reason we can't! Second, everyone seems to concede that height and weight "probably" make a difference. NHTSA then actually says that insufficient data exists to draw any general conclusions. Yet, they have no problem drawing general conclusions based on age! Third, and this is where it gets even more bizarre. At least in the data I was looking at, which may be old, I think it was from the 90's, it said that NO CHILD UNDER 9 had ever died from air-bag deployment. So, where does age 12 come from? Then, the pediatricians ring in, urging that laws be passed to require children under 12 to be in the rear seat "when available." THis makes NO SENSE. If its dangerous, its dangerous. Sitting in the front passenger seat with an airback doesn't suddenly become any safer because the three seats in the back are full! After much research, I guess I am left to conclude that it must be safe for a 9-12 year old to sit in the front seat if they have to. But it should not be so hard to find this answer, and I shouldn't have to feel like I am guessing.