How to update the property value in Revealing Prototype Pattern

This is my code example. I have a dog that with printAge() function that log the age property. age has also gave as public property.

let Dog = function(){

};

Dog.prototype = function(){
   let age = 3; 

   let printAge = function(){
    console.log(age);
   };

   return{
      age : age,
      printAge : printAge
   }
}();

Then I assign age from dog's instance and call printAge() to log but the value of age 3 is not updated with new value.

let DogInstance = new Dog();
DogInstance.age = 7;
DogInstace.printAge();

The result log is 3

But if I declare another function to assign the age instead of assinging age directly, it works.

Dog.prototype = function(){
   let age = 3; 
   let setAge = function(val){
     age = val;
   }
   let getAge = function() {
     return age;
   }
   let printAge = function(){
    console.log(age);
   };

   return{
      setAge  : setAge,
      printAge : printAge
   }
}();

let DogInstance = new Dog();
DogInstance.setAge(7);
DogInstace.printAge();

The result is: 7

Is that mean, we can't assign a property even if it was public in Revealing Prototype Pattern?