That is determined by the size, and build of a person, but yes they can legally sit in the front.
A six year old can be placed in the front seat but they must use a booster seat if they are under 135cm tall. Under no circumstances should a rear facing booster seat be used in the front seat.
A 9 year old child can ride in the front seat of a car. It is important that they are buckled in and can sit still.
Use your key on the left side by the seat. turn it counterclockwise. and the front seat should pull off. Once you remove the front seat there are two releases to remove the rear seat.
no
yes
No
yes
No
With a car seat to boost them up, Yes.
it is not the matter of age you can sit in the front seat when you are 1 year also.
In the UK yes, but if they are under 140 centimetres tall, they are required to use a booster seat/child seat. (But that goes for the back of the car, as well as the front).
The driver of the vehicle.