Users need to be 18 or older to register for Paid Memberships Pro Checkout

pmpro_age_check.php

<?php
/**
 * This will only allow users that are 18 years or older to signup on checkout.
 * This requires a custom 'date' field using Register Helper.
 * Add this code to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
 
 function custom_pmpro_validate_user_age( $okay ) {

	if( ! $okay ) {
		return $okay; //handle other errors first.
	}
	 
	if( '1' != $_REQUEST[ 'level' ] ) { //Change 1 to level ID that the CODE MUST APPLY TO, all other levels except 1 will not take the code into consideration.
		return $okay;
	}

	global $pmpro_msg, $pmpro_msgt;

	$date = $_POST[ 'date_of_birth' ];

	$year = intval( $date[ 'y' ] );
	$today = intval( date( 'Y' ) );

	if( $today - $year < 18 ){
		$pmpro_msg = "You must be older than 18 years old.";
		$pmpro_msgt = "pmpro_error";
		return false;
	}

	return $okay;
}
add_filter( 'pmpro_registration_checks', 'custom_pmpro_validate_user_age' );