We have a 12yr old and have been taking him on flights since he was about 7 months.  We have never had a problem.  We always change the dosages to take into account time differences.  And if you travel to some countries that need special vaccines children with CAH cannot be given live vaccines so they must carry a exemption certificate we found that out when traveling to Kenya and we also needed a letter to justify all the drugs we were carrying, we had to taken enough hydrocortisone to be able to double his dosage the entire time we are there.