There are several things wrong with the code, I made some comments where I corrected it. If you have any questions you can comment on this answer:
function Person(name, age) {
this.name = name;
this.age = age;
//var ageInTenYears = age + 10; //<--Why var, you can't
// use this anywhere but in the Person constuctor body
this.ageInTenYears=age+10;
}
Person.prototype = {
sayName : function(){
console.log(this.name);
},
sayNameAfterTimeOut : function(time) {
// check out this link about what this can be
// https://stackoverflow.com/a/19068438/1641941
var me=this;
setTimeout(function(){
me.sayName();
}, time);
},
sayAgeInTenYears : function() {
// you defined it as var so cannot use
// ageInTenYears outside the constructor body
//console.log(ageInTenYears);
console.log(this.ageInTenYears);
}
};
Person.prototype.sayAge = function() {
console.log(this.age);
};
Person.prototype.sayNameAndAge = function() {
console.log(this.name + this.age);
};
//just for good measure, someone may do
// Person.prototype.haveBaby=function(){
// return new this.constructor();
Person.prototype.constructor=Person;
var bob = new Person('bob', 30);
bob.sayName();
More on prototype, inheritance/mixin, overriding and calling super: https://stackoverflow.com/a/16063711/1641941