A.T. Robertson Commentary Luke 2:48

"And when they saw him, they were astonished; and his mother said unto him, Son, why hast thou thus dealt with us? behold, thy father and I sought thee sorrowing." — Luke 2:48 (ASV)

They were astonished (εξεπλαγησαν). Second aorist passive indicative of an old Greek word (εκπλησσω), to strike out, drive out by a blow. Joseph and Mary "were struck out" by what they saw and heard. Even they had not fully realized the power in this wonderful boy. Parents often fail to perceive the wealth of nature in their children.