Which one of the disciples was the youngest?
The Bible gives no specific ages for any of the disciples. Tradition says that John was the youngest.
In addition to other references on the net which suggest James and John being fishing in their father's boat when called to be disciples as a reason to think they were young, I'd add that he had a special place in Jesus' heart which might have been due to his youth. In addition, Jesus entrusted His mother's care to John at His crucifixion which also might have been due to his relative youth.