He is of age - Ἡλικιαν εχει, literally, he has stature, i.e. he is a full-grown man; and in this sense the phrase is used by the best Greek writers. See Kypke and Raphelius. Mature age was fixed among the Jews at thirty years.

But by what means he now seeth we know not,.... As to the third question they could say nothing to it, they were not present when the cure was wrought, and knew nothing of the matter, but what they had heard from their son, or from others, or both:
or who hath opened his eyes we know not; they had heard it was Jesus, and their son had doubtless told them it was he; but since they could say nothing of their own personal knowledge, they choose not to say anything of him:
he is of age; at man's estate, as, with the Jews, one was, who was at the age of thirteen years, if he could produce the signs of puberty: and such an one was allowed a witness in any case, but not under this age; nor if he was arrived to it, if the above signs could not be produced (q). This man very likely was much older, as may be thought from the whole of his conduct, his pertinent answers, and just reasoning: wherefore his parents direct the sanhedrim to him for an answer to their third question,
ask him, he shall speak for himself; or "of himself", as the Vulgate Latin and Ethiopic versions render it: their sense is, he is capable of giving an account of himself in this matter, and he will do it, and let him do it; put the question to him, and a proper answer will be returned; and so they left the affair to be issued in this way.
(q) Maimon. Hilchot Eduth, c. 9. sect. 7.