| Too young in my opinion, dont know exactly what the law states but guess the child needs to be wearing the right gear (but so does everyone) and be large/strong enough to hold on securely.
Kids vary so much in size you cant go by age alone, also a quick ride around the block has got to be different to riding along a motorway for a prolonged journey.
Its an interesting thread though as my nephew is ten and Im sure he is going to be asking soon.
Personally my first experience was riding a C90 aged 12, which of course I rode on the road !!! | 
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