Internal Constructors for Read Only Classes?

Let's say you're designing a class library, and you have a class like this:

public class Person
{
    public string Name { get; internal set; }
    public int Age { get; internal set; }
    public string Details { get; internal set; }
}

You're only ever returning Person objects from your API. There's no reason for the user of your API to ever change the properties, so you've made their setters internal, effectively making them read only.

Is there any point in leaving the default (public) constructor there? Why would an end user need to create an instance of your Person class if they can't change it? Would it make more sense to make the constructor internal too?

Or perhaps a protected constructor and property setters. That way if developers needed to create instances of Person for testing purposes (like mocking) then they could always derive from it.

Taking it one level further: Is there any reason to surface the class at all? Why not return an IPerson interface and make the entire class internal?

What do you think? Weigh in!