But how he can see now, or who opened his eyes, we don’t know. Ask him. He is of age; he will speak for himself.”
“He is of age” = he is an adult and capable of answering for himself. The Jews recognized adulthood as beginning at the age of thirteen and one day, but this expression used here probably means that the man is perfectly capable of thinking and speaking for himself. He is the one with first-hand knowledge of the incident; the parents have only the knowledge of what he has told them, which is the same information the Pharisees already have. This is a phrase used by Plato and the parents are within their rights, but they use emphatic pronouns when they refer to their son (he and him) and the emphasis demonstrates their determination to not get mixed up in this affair. They are obviously afraid and want to avoid the danger they perceive.