Exegesis:

The syntactic structure of v. 42 and v. 43 is complicated. These verses contain clauses, telling successively that when Jesus had reached the age of 12 years (a), his parents went up to the feast as usual (b), completed their stay (c) and went back (d), but that Jesus stayed behind (e) and his parents did not know (f). Of these 6 clauses (a) is necessarily a subordinate clause, indicating when the events described subsequently took place, and (e) and (f) are two co-ordinate independent clauses, containing the main information; the remaining 3 clauses (b), (c) and (d) are subordinate to (e), because Luke wants to focus all attention on the fact that Jesus stayed behind without his parents’ knowledge, which is the basis for the subsequent narrative. The necessary details which lead up to the focal clause hupemeinen Iēsous ‘Jesus stayed behind’ are passed over as quickly as possible: (b) and (c) are genitives absolute and (d) is a phrase in the articular infinitive (for a similar syntactic pattern cf. 3.21-22).

kai hote egeneto etōn dōdeka ‘and when he was twelve years old’; the genitive etōn dōdeka is genitive of quality, ‘(a boy) of twelve years.’ At the age of twelve a Jewish boy became a ‘son of the law’ and henceforth had to keep its enactments.

anabainontēn autōn kata to ethos tēs heortēs ‘when they went up according to the custom of the feast.’ autōn refers primarily to hoi goneis autou ‘his parents’ in v. 41 but may be taken to include Jesus. For kata to ethos tēs heortēs ‘according to the custom of the feast,’ cf. on 1.9. The genitive is one of association: the custom that is associated with the feast. The present tense of anabainontōn suggests that they were in the habit of going to Jerusalem every year, as is said explicitly in v. 41. anabainō (cf. on v. 4) is often a technical term for a pilgrimage to Jerusalem (cf. 18.31; 19.28; Mt. 20.17; Jn. 2.13; 5.1)

(V. 43) teleiōsantōn tas hēmeras ‘after having completed the days,’ of the feast. The phrase is, like the preceding one, in the genitive absolute, but the tense shifts from the present (suggesting habit, see above) to the aorist; this is done in order to describe an event which immediately precedes the event referred to by the main verb.

teleioō (also 13.32) ‘to complete,’ ‘to bring to an end’; tas hēmeras, ‘the days,’ without further specification, refers to a fixed period, probably the period of seven days prescribed in the law (cf. Ex. 12.15f; Lev. 23.6ff; Deut. 16.3).

en tō hupostrephein autous ‘when they were returning,’ articular accusative and infinitive, in the present tense because the event described is simultaneous with the event referred to by the main verb. autous refers to Joseph and Mary only. The verb here is best taken as durative, referring to a period that covers the first day of the journey.

hupemeinen Iēsous ho pais ‘the boy Jesus stayed behind.’

hupomenō ‘to stay behind,’ while others go away.

pais ‘boy,’ cf. on 1.54 and 2.17.

kai ouk egnōsan hoi goneis autou ‘and his parents did not know’; object of egnōsan is, of course, the fact that Jesus stayed behind.

Translation:

An attempt to imitate the syntactic pattern of the Greek may result in, ‘when his age was twelve years, after they had gone according…, and after they had completed-the-festival, at the going home the son Jesus still was in Jerusalem, but Joseph and his mother didn’t know’ (Javanese). In most receptor languages, however, such constructions are still more heavy than they are in Javanese, or even impossible. Therefore the syntactic pattern usually has to be altered, e.g. by starting a new sentence after clause (b) or clause (c). If the pattern of subordination has to be changed more thoroughly, it is important to mark the principal item of information clearly. To give a (probably purely theoretical) example: ‘In his twelfth year they went up according to the custom of the feast. They completed its days, and then returned. As for the boy Jesus, however, he stayed behind in Jerusalem, and/but his parents did not know it.’

He and the boy Jesus (v. 43) often can better change places, ‘The boy Jesus … he (or, the boy).’

They went up. That Jesus is included is not self-evident for non-Jewish readers; hence specifications of subject such as, ‘the-three-of-them’ (Malay), ‘they together’ (Balinese); cf. also ‘they-took-him-with-them-upward’ (Tae’ 1933). To express the specific meaning that went up has in this context some versions add a goal, e.g. “to the City” (Phillips), ‘to Jerusalem’ (Kituba, Balinese), ‘thither’ (Bible de Jérusalem, Bahasa Indonesia RC, KB); such an addition is based on translational, not on textual considerations. Cf. also “they made the pilgrimage” (New English Bible, taking ‘feast’ with this verb).

According to custom, preferably, “according to the custom of the feast” (cf. Exegesis), or, ‘as was customary at the feast,’ ‘as people used to do (or, which people always did) at the feast.’

(V. 43) And when the feast was ended, preferably ‘when they had completed the days (of the feast)’ (cf. Exegesis), or, ‘they remained (there) to the end (of the feast), or, during all the days (of the feast),’ cf. “and remained there for the days prescribed” (The Four Gospels – a New Translation).

As they were returning. Here the pronoun does not include Jesus; if “they” in v. 42 has been specified it may here be rendered, ‘his (or, the/Jesus’) parents,’ or be taken in an indefinite meaning, e.g. ‘when people were going home,’ cf. ‘at the returning’ (Balinese).

The boy Jesus, or, ‘the boy called Jesus’ (Sinhala), or simply, ‘the boy’; for the combination of noun and proper name cf. on v. 27. Boy indicates a further phase in Jesus’ growth, see on v. 17.

Stayed behind, i.e. remained in the place they had left, i.e. Jerusalem. The act is intentional and of his own doing, not happening by accident or through the agency of others (as suggested in some versions by a rendering that comes close to ‘was left behind’).

His parents did not know it, sometimes better subordinated to the preceding clause, cf. ‘without his parents knowing it’ (cf. Bahasa Indonesia, Batak Toba). Know here means ‘to be aware of,’ ‘to notice’ (Tagalog).

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.