Unit testing a method with Moq

I'm trying to learn how to do Unit testing with C# and Moq, and I've built a little test situation. Given this code:

public interface IUser
{

    int CalculateAge();
    DateTime DateOfBirth { get; set; }
    string Name { get; set; }
}

public class User : IUser
{
    public DateTime DateOfBirth { get; set; }
    string Name { get; set; }

    public int CalculateAge()
    {
        return DateTime.Now.Year - DateOfBirth.Year;
    }
}

I want to test the method CalculateAge(). To do this, I thought I should try giving a default value to the DateOfBirth property by doing this in my test method:

var userMock = new Mock<IUser>();
userMock.SetupProperty(u => u.DateOfBirth, new DateTime(1990, 3, 25)); //Is this supposed to give a default value for the property DateOfBirth ?
Assert.AreEqual(22, userMock.Object.CalculateAge());

But when It comes to the assertion, the value of CalculateAge() equals 0, although DateOfBirth equals new DateTime(1990, 3, 25).

I know this may look like a silly example, but whatever... I thought I could use mocking to give values to not-yet-developed method/properties in my objects, so the testing of a method wouldn't depend on another component of my class, or even setting up a default context for my object (hence the name of the user here...) Am I approaching this problem the wrong way?

Thanks.