I don't really know much about this kinda stuff, but i think that God's plan was to have Jesus born from a virgin.
If what TM says is true "Jewish men in Joseph's day generally married around the age of eighteen or twenty", then it makes perfect sense why God got Mary pregnant at an early age (before she got PG from somebody else)--plus she was already betrothed to Joseph.
I don't think Mary was less than 14 when that happened. I have been to catholic schools all my life and i've always been under the impression that she was 15.
but that's just me...
http://www.bozzysworld.com/images/stewie.jpg