pmpro_check_age.php

<?php
/**
 * add Register Helper
 */
function my_pmprorh_init()
{
	//don't break if Register Helper is not loaded
	if(!function_exists( 'pmprorh_add_registration_field' )) {
		return false;
	}
	
	//define the fields
	$fields = array();
	$fields[] = new PMProRH_Field(
        "check_age",             // input name, will also be used as meta key
        "text",                    // type of field
        array(
			"label" => "Age",
       "size" => 3,        // size attribute of text field
			'profile'	=> true,			// show in user profile
			'required'	=> true,			// make this field required
			'memberslistcsv' => true,
        )
    );


	
	//add the fields into a new checkout_boxes are of the checkout page
	foreach($fields as $field)
		pmprorh_add_registration_field(
			'checkout_boxes',				// location on checkout page
			$field						// PMProRH_Field object
		);
	//that's it. see the PMPro Register Helper readme for more information and examples.
}
add_action( 'init', 'my_pmprorh_init' );

function pmpro_check_age( $pmpro_continue_registration ) {
	
	//only bother if things are okay so far

	if(!$pmpro_continue_registration)
		return $pmpro_continue_registration;
	
	$check_age = $_REQUEST['check_age'];
	if ( !is_user_logged_in() ) {
	$onlyNumeric = filter_var($check_age, FILTER_SANITIZE_NUMBER_FLOAT, FILTER_FLAG_ALLOW_FRACTION);
	if ( $onlyNumeric < 18 ) {
		pmpro_setMessage( esc_html__( 'Must be 18+.' ), 'pmpro_error' );
		return false;
	}
	}
	
	return $pmpro_continue_registration;
	}


add_filter( 'pmpro_registration_checks', 'pmpro_check_age' );