Using hook_form_alter

I'd say use the hook that modifies the widget instead.

Would that be hook_field_widget_complete_form_alter? Unsure how to target that specific field that uses the select widget for datetime.

Does anyone have an example of modifying (any kind of) a profile field in the checkout pane? Thank you.

It actually sounds like you want the date field to so something fairly different to what it does OOTB.
What about a custom formatter? You can then set it up on a custom view mode, and IIRC you can select the view mode to use in the commerce checkout pane.

Thanks... I'm looking for something super simple - this works on the profile form itself:

case 'profile_registrant_edit_form':
    case 'profile_registrant_add_form':
      $form['field_date_of_birth']['widget'][0]['value']['#date_year_range'] = $max_age.':'.$min_age;

As this is an inline form, using hook_inline_entity_form_entity_form_alter works well.

<?php
/**
 * Implements hook_inline_entity_form_entity_form_alter().
 */
function MY_MODULE_inline_entity_form_entity_form_alter(&$entity_form, &$form_state) {
  // Alter the form to pre-fill and lock a couple fields.
  $is_target =  ($entity_form['#entity_type'] == 'profile'
    && $entity_form['#bundle'] == 'member_signup');
  if ($is_target) {
    $form['field_date_of_birth']['widget'][0]['value']['#date_year_range'] = $max_age . ':' . $min_age;
  }
}

Automatically closed - issue fixed for 2 weeks with no activity.