What does John 9:23 mean?

This repeats an earlier statement (John 9:21). The parents of a man Jesus healed of lifelong blindness (John 9:1–7) have testified before the local religious leaders (John 9:18). They clearly want no part of the controversy, which the prior verse helps to explain (John 9:22). These local leaders have promised to excommunicate those who accept Jesus' claim to be the Messiah. The healed man's parents respond with "no comment" as much as possible. Instead of speculating, they point out that their son is old enough to give his own legal testimony.

Of course, the formerly blind man has already been interrogated. During that first examination (John 9:13–17), the scribes and Pharisees rejected the man's claim that he'd been cured by Jesus. They had already made up their minds not to accept Jesus' message (John 5:39–40; 7:17). The meaning of the miracle was lost on them. From their perspective, there had to be some other way to explain what had happened.