Add a PMPro Register Helper conditional field to your checkout and profile screens.

pmpro-register-helper-conditional-field.php

<?php // Do not include in PMPro Customizations plugin
/**
 * Add a conditional Register Helper field with a checkbox that needs to be checked in order to show a Date field.
 */
function initialize_conditional_dob() {
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return false;
	}
	// define the fields
	$fields = array();

	/**
	 * Conditional field example. Date field showing depends on checkbox value
	 */
	$fields[] = new PMProRH_Field(
		'checking_condition',
		'checkbox',
		array(
			'label'   => 'Enter DOB',
			'text' => 'Enter your date of birth',
			'profile'   => true,
		)
	);
	$fields[] = new PMProRH_Field(
		'date_of_birth_2',
		'date',
		array(
			'depends' => array(
				array(
					'id' => 'checking_condition',
					'value' => true,
				),
			),
			'required'  => true,
			'label'     => 'Date Of Birth',
			'size'      => 40,
			'profile'   => true,
		)
	);

	foreach ( $fields as $field ) {
		pmprorh_add_registration_field(
			'checkout_boxes',
			$field
		);
	}

}
add_action( 'init', 'initialize_conditional_dob' );