I'm trying to get my head around what I call "optional features" but since I don't know their proper name in statistics I can't find any information about them. Essentially, I'm looking at a problem where sometimes some of the features do not even make sense. For example, if for each individual I have a feature "oldest son age" what happens if a particular individual does not have any son? How can one use this feature for regression or density estimation?
Sorry about the naive question. Just knowing what this kind of features are called in statistics or machine learning would be enormously helpful.