The law doesn't specifically refer to air bags except that it's illegal to carry a child in a rear-facing baby seat with an active air bag. While, in most situations, a child should not sit in a seat protected by an air bag, it's possible that it might be acceptable (or even advisable) for some older children, using a booster cushion, to occupy a seat with an air bag in front of it
in certain makes of car. The only rule is
'always read the car handbook' (or, of course, consult the manufacturer).
The law is solely concerned with 'appropriate restraints'. For children over 12 years of age, an adult seat belt is an 'appropriate restraint'. An adult seat belt also provides an 'appropriate restraint' for a child, under 12 years old, who is at least 135cm tall. In practice, this means that many 9-year-olds and most 10-year-olds can just use an adult seat belt.
When a child is under 12 years old and also under 135cm tall, the definition of an 'appropriate restraint' depends upon the child's weight. These are defined here:
http://www.childcarseats.org.uk/types/index.ht m
Chris