Why did jesus start his ministry at age 30?

Jesus was likely a member of the Jewish sect the Essennes (many of his teachings parallel theirs), and he was a rabbi. To be a rabbi back then, the process is somewhat different than now, but generally one is required to go through a rigorous training process, and one spends a long time as a disciple to other rabbis before they can become a rabbi themselves. It's a matter of training time. Jesus likely spent most of the thirty years as a disciple in training.