Adding a property to a Model in the ViewModel

With the Model

public class Person {
    public string Forename { get; set; }
    public string Surname  { get; set; }
    public string DOB      { get; set; }
}

I have a ViewModel which I'll be passing through to the View

public class PersonViewModel {
    public IQueryable<Person> PersonVM { get; set; }
    public string sometext{ get; set; }
}

If, for example, I wanted to calculate the age in the controller code and store it against each Person row in the IQueryable so it could be seen in the View, what's the best way of adding an Age property to each row?

I'm guessing that I don't have to include a fake property in the Person model like so

    public string Age      { get; set; }