How do I prepopulate the value of a taxonomy term reference field on the node creation form?
Try the following code.

function MYMODULE_form_MYFORM_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // Check if the node has a node ID. If not, then populate the fields with the default data.
  if (empty($form['nid']['#value']) &amp;&amp; (!isset($form['#node']-&gt;op))) {
    global $user;
    $user_fields = user_load($user-&gt;uid);
    // Set age.
    if (isset($user_fields-&gt;field_acc_age['und'][0]['tid'])) {
      $form['field_ad_age']['und'][0]['value']['#default_value'] = $user_fields-&gt;field_acc_age['und'][0]['tid'];
    }
  }
}