Determine the Age from DOB

For that you need to alter webform using hook_form_alter(&$form, &$form_state, $form_id) and apply below code for your fields( i.e. dob,age).

                $month = $form_state['input']['submitted']['dob']['month'];
                $day = $form_state['input']['submitted']['dob']['day'];
                $year = $form_state['input']['submitted']['dob']['year'];
                
                $date = new DateTime();
                if(!empty($year))
                $date = new DateTime("$year-$month-$day 00:00:00");
                $now = new DateTime();
                $interval=$date->diff($now);
                $form_state['input']['submitted']['age'] = $interval->y ;

Feel free to contact me incase of any query/concern regarding this.

Thanks!

For you purpose you require to place that calculation in js code. You can add the below code in your js file. Take two fields (dob, age).

jQuery( document ).ready(function( $ ) {        
        jQuery('.webform-component--dob').change(function($){
                month = jQuery('#edit-submitted-bod-month option:selected').text(); // Take your field value
                day = jQuery('#edit-submitted-bod-day option:selected').text(); // Take your field value
                year = jQuery('#edit-submitted-bod-year option:selected').text(); // Take your field value
                //alert(month+'month'+day+'day'+year+'year');
                if(jQuery(month.length != 0) && jQuery(day.length != 0)  && jQuery(year.length != 0)){
                        var today = new Date();
                        var yyyy = Number(today.getFullYear()); 
                        age = yyyy - year; 
                        jQuery('#edit-submitted-age').val(age); // Set  your age field value.
                }        
        });        
});

Feel free to contact me in case of any query/concern regarding the same.

Thanks!

Nop Vako! Not there, please place it in your theme js file (i.e./sites/all/themes/yourtheme/js/custom.js).

.webform-component--dob:- Please place your dob here.

Hope this helps you.

Over a year old; re-open if you still need help.