The oldest family of princes normally takes the succession. This means these people would go to great lengths to keep track of genealogies. All titles are recorded in the Velvet Tome, a sacred, ancient book held by the Imperial Scribes. Naturally, the head of the family alone benefits from the title's slow aging magic. Inheritance is a critical issue.
Gender never affects the mechanics of inheritance, age alone does.
The eldest scion always comes before the others. If not, then the first spouse of the dead noble can hold the title and receive its aging magic. If no spouse is alive at that time, the oldest sibling of the dead noble then holds the regency. If no sibling is alive, the emperor then names a regent. And so on.