Why did the blind man's parents say, "He is old enough. Ask him"?

Answer: Fear of being put out of synagogue

They feared the Jews, who had decided that anyone confessing Jesus as the Christ would be put out of the synagogue.

“His parents said this because they were afraid of the Jews. For the Jews had already determined that anyone who confessed Jesus as the Christ would be put out of the synagogue.”— John 9:22 (BSB)