ES5 Person Class

gistfile1.js

var Person = (function() {
  
  var __population = 0;
  
  function Person(opts) {

    var privateProps = ['dob'];
    
    //Private
    var props = {
      name: opts.name,
      dob: opts.dob,
      get age() {
        return new Date().getFullYear() - this.dob.getFullYear();
      }
    };
    
    //Priviledged
    this.get = function (prop) {
      return props[prop];
    };
    
    this.set = function (prop, value) {
      if(privateProps.indexOf(prop) < 0) {
        props[prop] = value;      
      }
    };
    
    Object.seal(this);
    Person.incrementPopulation();
  }
  
  Person.prototype = {
    //Public
    greet: function() {
      console.log('Hello my name is ' + this.get('name') +
                  ' and I am ' + this.get('age') + ' years old');
    }
  };
  
  Person.getPopulation = function() {
    return __population;
  };
  
  Person.incrementPopulation = function() {
    __population++;
  };
  
  Object.freeze(Person);
  return Person;
  
})();

var andy = new Person({
  name: 'Andy',
  dob: new Date('12/13/1979')
});

console.log(Person.getPopulation());

var dave = new Person({
  name: 'Dave',
  dob: new Date('03/09/1984')
});

Person.population = 100;
console.log(Person.population);
console.log(Person.getPopulation());

andy.set('dob', new Date('12/13/1990'));
andy.greet();
andy.set('name', 'Andrew');
andy.greet();
dave.greet();

Arguable all of the static method stuff is a bit wrong, a Factory would probably be a better way to handle the population control.