There is not an age restriction in the UK but they must keep to these three as long as:
1) They wear a correctly fitted, safety approved helmet. 2) They can comfortably reach the foot-pegs (and if that means raising the foot-peg mounting points to accommodate a child on the pillion seat, that is perfectly legal). 3) If they are under the age of 17, they must have a parent or guardian's consent to ride as a passenger on the motorcycle.
Thomas McKay-Smith