From my phsycology subjects uni it is around 11 to 12 that a childs so called value system is imprinted fully into thier conscious meaning propensity to take risk, what is right and wrong etc, however it is around this stage that they also begin to place more importance on their place within their peer heirachy which can lead to a compromise of the inbuilt value system i.e keeping up with the gang is more important that what is right or wrong.
At this age my concern would be not how they are riding but who they are riding with and what is thier values and do they match in with the behavours that you would expect out of your child
The above and also taking into account that at that age the fight or flight instinct will still take over from rational thought in a pressure traffic situation or emergency be your guide, so the question then is more around not what is the environment my child will be riding in but how will he or she react to sudden changes within thier environment
How childen react to situations should be more of a factor than age as an example are thier emergency stops at the moment a controlled reaction or is it still an eyes closed forget where the brakes are feet out scream fest if it is still the latter than i would suggest that more controlled riding in traffic is required
I preface this with my study of phsycology is limited and i am not a parent so ignore if yopu wish
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