Quote Originally Posted by Lord Condormanius
But not every female character turns into a princess at age 16, only the daughters of the faction ruler, while he is still in charge.

Actually daughters of the faction heir become princesses too. I find them to be of limited utility though so im not really bothered that they don't come along too often. I primarily use them on my low loyalty generals. I was using them to create alliances at first until I had a faction go to war with me a few turns after marrying one to a faction heir, poor girl is a widow now after her brother killed her husband in battle.