Hi Patrick,
DLA for children CAN be paid up to the age of 16, but this depends on the needs of the individual child. The guidelines on the DWP website recommend that by the age of 12, a child with diabetes on injections should need no more help or supervision than any other child their age, and therefore no more DLA will be paid. If the child uses an insulin pump, the age guideline is 14 to stop receiving DLA. However if you can provide evidence that your son DOES need more help and supervision than other 12 year olds without diabetes, he may be able to continue receiving DLA. Have you reapplied? Or did you apply and get refused? In which case, I would recommend appealing against their decision.