Show predefined value if selection is not made - Javascript

I have this javascript:

$(function() {

  $("#generate").click(function() {
  $("#id").val(generateID($("#age").val()));
    var increaseBy = increaseBase(generateSOCS());

    $("#numbersoc1").val(increaseBy(0));
    $("#numbersoc2").val(increaseBy(1));
    $("#numbersoc3").val(increaseBy(2));
    $("#numbersoc4").val(increaseBy(3));
  });

  $.getJSON("age.json", function(json) {

        $("#age").empty();
        $("#age").append($('<option>').text("Select age"));

        $.each(json, function(i, obj){
            $("#age").append($('<option>').text(obj.age).attr('value', obj.permanent));
        });
    });
});

function increaseBase(base) {
  return function(value) {
    return "SOCS" + (base + value).toString(16).toUpperCase()
  }
}

function generateSOCS() {
  return random(10, 16777215);
}

function generateID(permanent) {
    if(permanent == "Select age")
        return "Please select your age first!";
        var add = 5
    return (permanent + random(1,100) + random(100,50000) + ++add);
}

function random(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

And this html:

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src='script.js'></script>
<h1>Number Generator</h1>

<div>
 <select name="age" id="age"></select><br/>

  <input id="generate" type=button value="Generate" /><br/><br/> 
  <br/>
  Identifier<br/>
   <input id="id" placeholder="Identification Number" /><br/><br/>
  Number<br/>
  <input id="numbersoc1" placeholder="Number Soc. S1." /><br/>
  <input id="numbersoc2" placeholder="Number Soc. S2." /><br/>
  <input id="numbersoc3" placeholder="Number Soc. S3." /><br/>
  <input id="numbersoc4" placeholder="Number Soc. S4." /><br/>
</div>

And this json:

[
    { "age": "18", "permanent": "AA" },
    { "age": "19", "permanent": "BB" },
    { "age": "20", "permanent": "CC" }
]

When I click the 'Generate' button without selecting an age in the drop down menu then the 'Identifier' box will show the following: 'Please select your age first!' which is normal and expected however I would like to do the same thing for the 4 Number boxes. Help would be appreciated.

Also, can the code be made more efficient? By that I mean shorter. If yes how?

Thank you in advance.

You should check the value of the selected value in #age first.
This can be achieved with a simple if else statement.

$("#generate").click(function() {
  if ($('#age').val() !== '<your default value>') {
    //your code for setting the values of the #numbersoc* inputs
  } else {
    //code that notifies the user about needing to select an age.
  }
});

Note that for this example it is required to have a default value as an option in your age select tag.
this can be achieved by using the following option tag.

<option selected="true" disabled="disabled">Select your age</option>  

Edit
added the solution into OP's code.

$(function() {
  $("#generate").click(function() {
  //checks if the value of #id is an empty string
    if ($("#id").val(generateID($("#age").val())).val() == '') {
    //alerts the user that they should enter an age
    //feel free to make something nice here
      alert('Please insert age first');
    } else {
    var increaseBy = increaseBase(generateSOCS());

    $("#numbersoc1").val(increaseBy(0));
    $("#numbersoc2").val(increaseBy(1));
    $("#numbersoc3").val(increaseBy(2));
    $("#numbersoc4").val(increaseBy(3));
  }
});

  //uncomment this when using the json
  /*$.getJSON("age.json", function(json) {

    $("#age").empty();
    $("#age").append($('<option>').text('Select age').attr('value', 'Select age')
    .attr('selected', true).prop('disabled','disabled'));

    $.each(json, function(i, obj) {
      $("#age").append($('<option>').text(obj.age).attr('value', obj.permanent));
    });
  });*/
});

function increaseBase(base) {
  return function(value) {
    return "SOCS" + (base + value).toString(16).toUpperCase()
  }
}

function generateSOCS() {
  return random(10, 16777215);
}

function generateID(permanent) {
// simple check for default, empty or null value
  if ($('#age').val() == 'Select age' || $('#age').val() == '' || $('#age').val() == null) {
    return;
  }
  var add = 5
  return (permanent + random(1, 100) + random(100, 50000) + ++add);
}

function random(min, max) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src='script.js'></script>

<h1>Number Generator</h1>

<div>
  <select name="age" id="age">
  <!-- the options will be auto generated -->
    <option selected="true" disabled="disabled">Select age</option>
    <option>1</option>
    <option>2</option>
    <option>3</option>
  </select>
  <br/>

  <input id="generate" type=button value="Generate" />
  <br/>
  <br/>
  <br/> Identifier
  <br/>
  <input id="id" placeholder="Identification Number" />
  <br/>
  <br/> Number
  <br/>
  <input id="numbersoc1" placeholder="Number Soc. S1." />
  <br/>
  <input id="numbersoc2" placeholder="Number Soc. S2." />
  <br/>
  <input id="numbersoc3" placeholder="Number Soc. S3." />
  <br/>
  <input id="numbersoc4" placeholder="Number Soc. S4." />
  <br/>
</div>