I checked out
http://www.nhtsa.dot.gov/CPS/
and you have to read it carefully "All children aged 12 and under
should go in the back. This is advisory. As far as i'm aware there are no laws regarding ages of front seat sitting people. The only legal requirement to my knowledge is that the driver and all passengers, have some form of safety restraint. A baby for example, is seated in a baby carrier (fitted to face away form the dashboard), a young child from about 12-18 months onwards in a child seat (if suitable), children from about 2-4 years in a booster seat. These are all advisory measures and vary with the size of child (mine for example outgrew his baby carrier after about 6 months). I am not sure but I think there is a minimum requirement that children under 4 not use standard seatbelts.