Use variable outside the success function from an ajax/jquery call

 var test; // <-- (1) This code runs first  
 $.ajax({  // <-- (2) Then this runs  
    type: "GET",
    url: "../views/person/controller.php?actor=person&action=checkAge",
    data: "age=" + value,
    success: function(msg){
        console.log(msg); //<-- (4) Finally this is run. IF your request is a success 
        test = msg; 
    },
 });
 Validate.fail(test); // <-- (3) This runs third  

Look at the order in which the code runs. Your variable is simply not available at that point because it's running when the code is triggered via the callback

Probably because Validate.fail(test) occurs immediately after the asynchronous call. Remember it is ASYNCHRONOUS, meaning it executes parallel to javascript running on your page.