My most recent game as England:

William the Conquerer is the king at the start of the campaign. His heir is his oldest son, Prince Rufus. Over the course of the early game, Rufus adopts two men (early conquests) and then has two natural sons. At the time William dies, Rufus becomes king and the new heir is the first of Rufus' adopted children. The prince doesn't have any children for a while, so when Rufus dies his son is only 13. The prince becomes the king, but the new heir is now Rufus fourth child. It skipped over both the second adopted son and the oldest of the two natural sons.