Magento – Magento 2: How to add date of birth and gender fields in checkout

This usually happens when you specify "system" => "1" for the customer attributes. In order to make this working, you may create an upgrade script to set "system" => "0".

Here's an example:

app/code/YourVendor/YourModule/Setup/UpgradeData.php

use Magento\Framework\Setup\UpgradeDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;
use Magento\Customer\Setup\CustomerSetupFactory
use Magento\Customer\Setup\CustomerSetup;
use Magento\Customer\Model\Customer;
...

public function __construct(
    CustomerSetupFactory $customerSetupFactory
) {
    $this->customerSetupFactory = $customerSetupFactory;
}
...

public function upgrade(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
{
    if (version_compare($context->getVersion(), '0.2.0') < 0) {
        $customerSetup = $this->customerSetupFactory->create();
        $customerSetup->getEavConfig()
            ->getAttribute(Customer::ENTITY, 'example_attribute_code')
            ->setData('is_user_defined', 1)
            ->setData('system', 1)
            ->save();
    }
}

Create a gender.phtml file under

app/design/frontend/Your_Theme/default/Magento_Customer/templates/widget/

and add the following code:

<div class="field gender<?php if ($block->isRequired()) echo ' required' ?>">
    <label class="label" for="<?php /* @escapeNotVerified */ echo $block->getFieldId('gender')?>"><span><?php /* @escapeNotVerified */ echo __('Gender') ?></span></label>
    <div class="control">
        <?php $options = $block->getGenderOptions(); ?>
        <?php $value = $block->getGender(); ?>
        <?php foreach ($options as $option): ?>
            <?php if(!$option->getValue()) continue; ?>
            <div class="field choice">
                <input type="radio"
                       class="radio"
                       id="gender-option-<?php /* @escapeNotVerified */ echo $option->getValue() ?>"
                    <?php if ($block->isRequired()) echo 'data-validate="{\'validate-one-required-by-name\':true}"'?>
                       name="<?php /* @escapeNotVerified */ echo $block->getFieldName('gender')?>"
                       data-selector="<?php /* @escapeNotVerified */ echo $block->getFieldName('gender')?>"
                    <?php if ($option->getValue() == $value) echo ' checked="checked"' ?>
                       value="<?php /* @escapeNotVerified */ echo $option->getValue() ?>"/>
                    <label class="label"
                       for="gender-option-<?php /* @escapeNotVerified */ echo $option->getValue() ?>">
                    <span><?php /* @escapeNotVerified */ echo $option->getLabel() ?></span>
                </label>
            </div>
        <?php endforeach;?>
    </div>
</div>

This will display the gender field in Customer Registration and in customer's account Edit Account Information as radio. Hope that helps!