jquery - Input from 2 different forms in Javascript Validation function -

i using following design in order validate age input (must greater 18):

the form following:

<div class="col-md-2 form-group">             <div class="input-group date">                 <form:input path="legal_resp.birth_date" class="form-control datepicker" placeholder ="ηη/µµ/εεεε" data-validbirthdate="true"/>                 <span class="input-group-addon btn">                 <span class="glyphicon glyphicon-calendar"></span></span>             </div>              <div class="help-block with-errors"></div>             <form:errors path="legal_resp.birth_date" cssstyle="color: #ff0000;"/>         </div> 

the options pass argument .validator following:

var options={       custom: {              validbirthdate: function (input) {                 var str= input.val();                 if(str == '')                     return true;                 var parts = str.split('/');                  if (parts.length < 3)                     return false;                 else {                     var day = parseint(parts[0]);                     var month = parseint(parts[1]);                     var year = parseint(parts[2]);                     if (isnan(day) || isnan(month) || isnan(year)) {                         return false;                     }                     if (day < 1 || year < 1)                         return false;                     if(month>12||month<1)                         return false;                     if ((month == 1 || month == 3 || month == 5 || month == 7 || month == 8 || month == 10 || month == 12) && day > 31)                         return false;                     if ((month == 4 || month == 6 || month == 9 || month == 11 ) && day > 30)                         return false;                     if (month == 2) {                         if (((year % 4) == 0 && (year % 100) != 0) || ((year % 400) == 0 && (year % 100) == 0)) {                             if (day > 29)                                 return false;                         } else {                             if (day > 28)                                 return false;                         }                           }                      var d = new date(year, month-1, day);                      var agedifms = date.now() - d.gettime();                     var agedate = new date(agedifms); // miliseconds epoch                     var age = math.abs(agedate.getutcfullyear() - 1970);                      if(age<18)                         return false;                     return true;                        }                 } 

i have following question:

i want exted function in order take input 2 different forms (2 age inputs) , compare them. understand need argument function. how can achieve this?