Since Joseph's irrational and personal fear of Archelaus was unlikely to dissipate while the ethnarch still ruled in Judea, I believe we must conclude that Jesus was twelve years old in March of 7 AD. That spring was the first Passover Joseph could bring his young charge safely into Judea after Archelaus had left (summer 6 AD). This means Jesus would have turned 13 any time in the twelve months after March, 7 AD. That same math also puts Jesus' birth anywhere in the twelve months prior to April of 6 BC - so as early as May of 7 BC.
Without a doubt, there are plausible explanations to account for the many chronological ramifications of this conclusion. (Especially in 7 BC.) But it should be strongly considered that analysis of Luke 2:42 in the light of Matthew 2:22 grants us a historical basis for positing the age of Jesus during one actual point in time, which is a far sight better than basing estimates of the Lord's birth on pure speculation, sketchy astronomical interpretations or (the ever popular) "sometime before 4 BC".
If scholars come to consider this a solid historical connection, then it should prove helpful as an improved starting point for New Testament Chronology. The alternative is to continue working from "about 30" and the "fifteenth year of Tiberius", data about which (unfortunately) there is far too much 'wiggle room'. Theories on the census and star of bethlehem will always require speculation, but the only resistance from what we know of Jesus' ministry years will be some unnecessarily staunch definitions of the word "about". Surely, all christian doctrines and views of scripture are flexible enough to swallow a 4 to 6 year difference in Luke 3:23. (Also, we'll never know when the "46 years" of John 2:20 began, because we don't know how many years of prep-work was needed before the Jews would allow Herod to begin actual construction! See Josephus' Antiquities' 15:388-390.)
For all these reasons, I hope scholars will begin giving extra attention to the historical view of Luke 2:42 in connection with Matthew 2:22. Besides, May of 7 BC begins a count of forty years (inclusively) up to just before Pentecost of 33 AD. Not that numerology has anything to do with history, but I hope it might inspire some to take a closer look. ;)