What makes you entitled to princess or prince in Vampire Academy?

Any Moroi who are of age (eighteen or older) are lords or ladies in their families. For instance, I saw Christian get referred to in Rose's thoughts as Lord Ozera because he is eighteen in the book it happens in and she is reminding herself that as a guardian, she'll have to start calling him that instead of Christian. I imagine that when a dhampir has to be proper or speak to a royal Moroi, they call them Lord or Lady whatever-their-name-is.

To become the prince or princess of a royal family, you have to be the oldest living person in it (though the title will pass on to the next oldest person in the family if the oldest person becomes queen or king). Lissa was called princess in most of the books because as the last Dragomir alive, she was also technically the oldest living Dragomir and got the title.