DOB.PHTML

<label for="<?php echo $this->getFieldId('month')?>"<?php if ($this->isRequired()) echo ' class="required"' ?>><?php if ($this->isRequired()) echo '<em>*</em>' ?><?php echo $this->__('Date of Birth') ?></label>
<div class="input-box customer-dob">
    <?php
    // days
    echo "<select id=\"sDay\" name=\"sDay\" rel=\"".str_replace(":","\:",$this->getFieldId('day'))."\" class=\"dateselect\">";
    $selected = (!$this->getDay()) ? "selected=\"selected\"" : "";
    echo "<option value=\"\" ".$selected.">Day</option>";
    for ($i = 1; $i <= 31; $i++) {
        $ti = str_pad($i, 2, "0", STR_PAD_LEFT);
        $selected = ($this->getDay() == $ti) ? "selected=\"selected\"" : "";
        echo "<option value=\"".$ti."\" ".$selected.">".$i."</option>";
    }
    echo "</select>";
    ?>
   
    <?php
    // months
    $months = array(
        '01'=>'January',
        '02'=>'February',
        '03'=>'March',
        '04'=>'April',
        '05'=>'May',
        '06'=>'June',
        '07'=>'July',
        '08'=>'August',
        '09'=>'September',
        '10'=>'October',
        '11'=>'November',
        '12'=>'December',
    );
    echo "<select id=\"sMonth\" name=\"sMonth\" rel=\"".str_replace(":","\:",$this->getFieldId('month'))."\" class=\"dateselect\">";
    $selected = (!$this->getMonth()) ? "selected=\"selected\"" : "";
    echo "<option value=\"\" ".$selected.">Month</option>";
    foreach ($months as $key=>$var)
    {
        $selected = ($this->getMonth() == $key) ? "selected=\"selected\"" : "";
        echo "<option value=\"".$key."\" ".$selected.">".$var."</option>";
    }
    echo "</select>";
    ?>
   
    <?php
    // years
    $year_end = date('Y');
    $year_start = $year_end-99;
    echo "<select id=\"sYear\" name=\"sYear\" rel=\"".str_replace(":","\:",$this->getFieldId('year'))."\" class=\"dateselect\">";
    $selected = (!$this->getYear()) ? "selected=\"selected\"" : "";
    echo "<option value=\"\" ".$selected.">Year</option>";
    for ($i = $year_end; $i >= $year_start; $i--) {
        $selected = ($this->getYear() == $i) ? "selected=\"selected\"" : "";
        echo "<option value=\"".$i."\" ".$selected.">".$i."</option>";
    }
    echo "</select>";
   
    ?>
    <div class="dob-full" style="display:none;">
        <div class="dob-day"><input type="hidden" id="<?php echo $this->getFieldId('day')?>" name="<?php echo $this->getFieldName('day')?>" value="<?php echo $this->getDay()?>" /></div>
        <div class="dob-month"><input type="hidden" id="<?php echo $this->getFieldId('month')?>" name="<?php echo $this->getFieldName('month')?>" value="<?php echo $this->getMonth()?>" /></div>
        <div class="dob-year"><input type="hidden" id="<?php echo $this->getFieldId('year')?>" name="<?php echo $this->getFieldName('year')?>" value="<?php echo $this->getYear()?>" /></div>
        <input type="hidden" id="<?php echo $this->getFieldId('dob')?>" name="<?php echo $this->getFieldName('dob')?>" />
    </div>
    <div class="validation-advice" style="display:none;"></div>
</div>
<script type="text/javascript">
//<![CDATA[

var customer_dob = new Varien.DOB('.customer-dob', <?php echo $this->isRequired() ? 'true' : 'false' ?>, '<?php echo $this->getDateFormat() ?>');
jQuery(document).ready(function() {
    // on load set field values
    jQuery( ".dateselect" ).each(function() {
      related_field_id = jQuery(this).attr('rel');
      current_value = jQuery( "#"+related_field_id ).val();
      jQuery(this).val(current_value);
    });
      jQuery( "#<?php echo str_replace(":","\:",$this->getFieldId('dob'));?>" ).val(jQuery("#<?php echo str_replace(":","\:",$this->getFieldId('day'));?>").val()+'/'+jQuery("#<?php echo str_replace(":","\:",$this->getFieldId('month'));?>").val()+'/'+jQuery("#<?php echo str_replace(":","\:",$this->getFieldId('year'));?>").val());
    // on change set field values
    jQuery( ".dateselect" ).change(function() {
      current_value = jQuery(this).val();
      related_field_id = jQuery(this).attr('rel');
      jQuery( "#"+related_field_id ).val(current_value);
      jQuery( "#<?php echo str_replace(":","\:",$this->getFieldId('dob'));?>" ).val(jQuery("#<?php echo str_replace(":","\:",$this->getFieldId('day'));?>").val()+'/'+jQuery("#<?php echo str_replace(":","\:",$this->getFieldId('month'));?>").val()+'/'+jQuery("#<?php echo str_replace(":","\:",$this->getFieldId('year'));?>").val());
    });
});

//]]>
</script>
http://www.endreywalder.com/blog/change-date-of-birth-field-to-select-boxes-in-magento/