3 Answers

The California Law:

A child under the age of six weighing less than 60 pounds may ride in the front seat of a vehicle when: 

There is no rear seat or the rear seats are either side-facing jump seats or rear-facing seats. 

The child passenger restraint system cannot be installed properly in the rear seat.

All rear seats are already occupied by children under the age of 12 years. 

A medical reason requires the child to ride in the front seat.

A child may not ride in the front seat of an airbag-equipped vehicle if the child: 

Is under one year of age. 

Weighs less than 20 pounds. 

Is riding in a rear-facing child passenger restraint system.

Children ages six and older but less than 16 and children under age six who weigh more than 60 pounds, must be properly secured in an appropriate child passenger restraint system or safety belt which meets federal safety standards.

In Australia the child must be in a recommended safety standard child booster seat up until the age of eight,and placed in the back seat.

Not sure, but there is an age requirement.