Require accounts to be 18 years or older, in order to checkout and signup for a membership level.

pmpro-min-age-checkout.php

<?php
/**
 * Require age verification before being able to signup for a membership level.
 * Create a Date field in User Fields and rename the $dob_field_name and adjust the $min_age values.
 * This supports PMPro V3.2+ and has backwards compatibility, please read the comments.
 *
 * To add this customization to your site, please follow this guide - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_min_required_age( $okay ) {

    $dob_field_name = 'birthday'; // The name of the user field that contains the date of birth.
    $min_age        = 18; // The minimum age required to register.

    // Bail if already not okay
    if ( ! $okay ) {
        return $okay;
    }

    if ( isset( $_REQUEST[ $dob_field_name ] ) ) {
        $birthday = $_REQUEST[ $dob_field_name ]; // This is a string,
    } else {
        pmpro_setMessage( 'Please fill out the ' . $dob_field_name . ' field.', 'pmpro_error' );
        return false;
    }

	// Convert Birthday to an array if it is not already.
	if ( ! is_array( $birthday ) ) {
		$birthday = explode( '-', $birthday );
		$birthday = array_combine( ['y', 'm', 'd'], $birthday );
	} 
	
    // Check if each part of the date is numeric and if it forms a valid date.
    if ( ! isset($birthday['m'], $birthday['d'], $birthday['y']) || 
         ! is_numeric( $birthday['m'] ) || 
         ! is_numeric( $birthday['d'] ) || 
         ! is_numeric( $birthday['y'] ) || 
         ! checkdate( $birthday['m'], $birthday['d'], $birthday['y'] ) ) {
        pmpro_setMessage( 'Please enter a valid ' . $dob_field_name . '.', 'pmpro_error' );
        return false;
    }

    try {
        $now = new DateTime(); // Get the current date.
        $dob = new DateTime( $birthday['y'] . '-' . $birthday['m'] . '-' . $birthday['d'] ); // Get the date of birth.
        $age = $dob->diff( $now )->y; // Calculate the age of the user.
    } catch (Exception $e) {
        pmpro_setMessage( 'There was an error processing the date. Please try again.', 'pmpro_error' );
        return false;
    }

    // Check if the user is at least 18 years old.
    if ( $age < $min_age ) {
        pmpro_setMessage( 'You must be at least ' . $min_age . ' years old to register.', 'pmpro_error' );
        return false;
    }

    return $okay;
}
add_filter( 'pmpro_checkout_user_creation_checks', 'my_pmpro_min_required_age' );
// add_filter( 'pmpro_registration_checks', 'my_pmpro_min_required_age' ); // Use this for all PMPro < 3.2 installations.