你如何调用从另一个类继承的方法？

此代码无法在最后一行运行.我不知道为什么.

var Vehicle = function() {
	var age = 21;			//private variable
	this.setAge = function(age2) {age = age2;};
	this.getAge = function() {return age;};
};

var Plane = function() {};
Plane.prototype = Object.create(Vehicle.prototype);

var plane = new Plane();
console.log( plane instanceof Vehicle );
//console.log( plane.getAge() );	//TypeError: plane.getAge is not a function

var Plane = function() {};

var Plane = function ( ) {
    Vehicle.call( this );
};