First, you'll need to find..
includes/templates/template_default/templates/tpl_modules_create_account.php
Make a copy of it and put it into your custom template. Then in that file, find..
PHP Code:
<?php
if (ACCOUNT_DOB == 'true') {
?>
<fieldset>
<legend><?php echo TABLE_HEADING_DATE_OF_BIRTH; ?></legend>
<label class="inputLabel" for="dob"><?php echo ENTRY_DATE_OF_BIRTH; ?></label>
<?php echo zen_draw_input_field('dob','', 'id="dob"') . (zen_not_null(ENTRY_DATE_OF_BIRTH_TEXT) ? '<span class="alert">' . ENTRY_DATE_OF_BIRTH_TEXT . '</span>': ''); ?>
<br class="clearBoth" />
</fieldset>
<?php
}
?>
Move the section of code above to a different part of the file, wherever you want it displayed.
Then find includes/languages/english.php and edit any relevant define statements. The Developer's Tool Kit should prove to be useful here. Don't forget to also overide this file.
You may find that you run into validation problems doing the above. Zen Cart might expect something in the form of a date to be entered. In that case, you would also need to modify some JavaScript.
There'll be some admin language files that will require some editing as well. Again, make use of the Developer's Tool Kit.