NO NO NO
The main reason a younger child cannot sit in the front of a vehicle is due to the airbag which could cause significant injury or even death to a small child. Generalizing an age limit of 13 ensures that most kids will be big enough to withstand the forces of the airbag during a collision (big enough meaning that their skeletal development is advanced enough, they are tall enough, etc. Many factors other than just height and weight). Studies have shown that children over the age of 13 benefit from the protection of an airbag, whereas younger children often suffered injury by airbags, which is why they give an age rather than height and weight limits. A vehicle with no airbag could have a child ride in the front providing they still use the appropriate car seat or booster (if needed), although the rear seat is still the safest place for them to be.