[dismantled]

function Human(name, sex, lang, age) {
  this.name = name;
  this.sex = sex;
  this.lang = lang;
  this.age = age;
}

Human.prototype.eldest = function(humans) {
	var maxAge = 0;
	humans.forEach(function(human) {
  	if (human.age > maxAge)
    	maxAge = human.age;
  });
	alert(maxAge);
};

var human = new Human();
var humans = [];
humans.push(new Human("Ken", "male", "eng", 28));
humans.push(new Human("Pol", "male", "eng", 22));
humans.push(new Human("July", "female", "uk", 25));
humans.push(new Human("Bill", "male", "rus", 36));
  
human.eldest(humans);

Посмотреть в действии можно тут: https://jsfiddle.net/8u63xku6/