Assuming that the DOB field uses the css class, dob, and the Age field uses the css class, age, you can use the following Javascript

  $('.dob input').on('change', function() {
    var today = new Date();
    var birthDate = new Date($('.dob input').val());
    var age = today.getFullYear() - birthDate.getFullYear();
    var m = today.getMonth() - birthDate.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
        age--;
    }
    $('.age input').val(age);
  });