Exegesis:

existanto de pantes hoi akouontes autou ‘all who heard him were amazed’; the clause is the last part of the description of the situation which Mary and Joseph found. This is brought out by the imperfect tense of existanto, ‘were amazed,’ as compared with the aorists in v. 46 and v. 48.

existamai ‘to be amazed.’

epi tē sunesei kai tais apokrisesin autou ‘at his insight and his answers.’ The latter word implies that Jesus did not only ask questions but was also questioned; this supports the idea of a discussion at which also eperōtōnta ‘asking questions’ in v. 46 hints (cf. last note on v. 46).

sunesis ‘insight,’ here best taken as referring to both questions and answers.

apokrisis (also 20.26) ‘answer.’

Translation:

All who heard him, or, .’.. heard his words,’ cf. on 1.41.

Were amazed at, or more positively, ‘admired’; for the verb cf. on “wondered at” in 1.21.

His understanding and his answers. Though the two terms are in themselves neutral the intention in this context is clearly positive; hence the first noun may be rendered by, ‘his insight,’ “his intelligence” (New English Bible), “his quick understanding” (Knox), ‘his cleverness’ (Tae’ 1933), ‘the acuteness of his mind’ (Batak Toba); and the second by, ‘his wise answers,’ ‘the rightness of his answers’ (Tae’ 1933). Shifts to a verbal clause may result in something like, ‘the way how he understood and answered,’ ‘the fact that (or, because) he understood things very well and answered wisely.’ For to understand a figurative rendering is sometimes used, e.g. ‘to receive in the heart,’ ‘to see clearly,’ ‘to reach (or, find, or, hear) with the mind/heart.’

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.