There is a lot to consider there:
- Do you live in a town, city or village?
- Is your son worldly wise?
- What is the traffic like on the roads he will be travelling?
- Are you concerned about him getting in an rta or being abducted? (Sorry to be blunt)
- Does he have a mobile or have you got tracking software so you can track/find him if you need to?
- Does he have a Garmin to follow a route? Does he know the area if he gets lost and the Garmin freezes?
- Will there be a support car (ie you or Mum perhaps) available if he gets in trouble?
- Are his mates sensible and do they have the same ability as him?
- Could you club together with other parents and take it in turns to chaperone? Maybe 2 dedicated chaperones could set out on the same route an hour later to act as a broom wagon.
- Are there any cycling clubs that he could do group rides with?
My youngest is 14 yo and 6ft 1' and I still worry when he is off on his mtb with his mates a few miles from home! Twelve is a bit young imho, but, there are no rules, its what you feel comfortable with. I think the minimum I would insist on is a fully charged phone, a gps tracker and an available support car that could get to him quickly if there was a problem.