Hi stephenadmeeker, I realize that you asked this question around a month ago, so you probably already know the answer by now. But I figured I'd provide an answer here in case anyone else is reading this.
A variable only needs to be defined once. That's what you did in your first line:
var userAge = prompt("What's your age, user?");
When defining a variable, you don't even have to assign it a value (though in this case, it does make sense to do so). For example, you can write:
The userAge variable is now defined, and we can use it later on when we want to.
If you need to assign a different value to the same variable, you don't have to define it again (var ). You just have to assign or prompt for a new value, like this:
userAge = prompt("Whats your age, user?")
Hope that helps.