Get-Only Properties (managing access to properties)

Why would you anyone make a property inaccessible?

public int age;
public int Age 
    {
      get{return age;}
      private set{age=value;}
    }

Also, the second thing is an exercise says “Say we want programs to get the value of the property, but we don’t want programs to set the value of the property”

How is it possible to get the value out of a property without setting it in the first place?

Thanks in advance.