PG-13
A PG-13 rating might come as a surprise in light of all of those Luke-based G-rated Christmas pageants and Sunday school lessons out there. But read carefully and you'll see that Luke is full of references to violence and raises many very complicated, mature, and adult-oriented themes.
Without parental guidance, children and younger teenagers might not be able to properly process, for example, the nobleman's order to bring forth his enemies and slaughter them while he watches in the Parable of the Ten Pounds (19:27) or the rich man's agony in Hades, where he is tormented in flames (16:24-25). Add this to the destruction of Jerusalem, the Jewish rejection of Jesus, the necessity of the crucifixion, divine punishment… and you've got yourself a PG-13.