Is there an age restriction for children sitting in the front seat? My child is 11 and always sits in the back, but at what age can he sit in the front without harm from the airbag, etc?
...should be your vehicle's owners manual. The manufacturer that designed the system should have a complete set of recommendations to follow there for you.
Sitting in the back seat is not as nice as when you can sit up front and see what is going on around you more easily, but the center position of the rear seat is probably the safest place to be when the vehicle is involved in a collision. You've made a very wise choice when you have your children sit there.
There is no age restriction for sitting in the front seat in BC traffic law. The age restrictions all relate to what kind of restraint is used and who is responsible for making sure that they are used. Seat belts are dealt with in Section 220 of the Motor Vehicle Act and child restraints in Division 36 of the Motor Vehicle Act Regulations. There are also rules about what a proper seating position is in Division 39 of the Motor Vehicle Act Regulations.
There are many reputable sources of information on the web concerning seating and child restraint. Car Seats, Seat Belts and Your Child on Transport Canada's web site is a good place to start. They suggest that children aged 12 and under should ride in the back seat. However, child body size is probably a better indicator than age. If your child cannot sit with their back against the seatback and their legs bent comfortably at 90 degrees over the front of the seat, they are probably still too small to use the front seat safely.
Another good resource is the Child Passenger Safety web site.