Luke 15:12

“And the younger of them said to his father, ‘Father, give me the portion of your substance which falls to me.' And he divided to them his living.”

The younger son come to his father with the request that he might have his share of what he would in the future inherit. In a case where there were two sons this would normally be one third of the whole (the elder brother who would take over responsibility for dependants would receive a double portion), although in a situation like this where it was received early it may have been a lesser proportion (for the whole see Deuteronomy 21:17). Such an apportioning of an inheritance before death did happen regularly, and the principle behind it was that the sons would then have financial responsibility towards their father who retained a right to receive the income, and utilise the capital. But that a son would actually request it while his father was in good health would be an unusual case, and is probably intended to emphasise the waywardness of the son and the goodheartedness of the father. There was probably no thought at this stage of the younger son leaving home, except for business reasons, nor of him having the capital simply to do what he liked with. The younger son was probably only in his late teens, for he was unmarried, and had seemingly no thoughts of marriage.