Check that a user has filled in a valid birthday date and that the date of birth falls inside a required set date range for the level they are checking out.

pmpro-required-age-range.php

<?php
/**
 * Require a specific age range for some membership levels.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_required_age_range( $okay ) {

	// Set the age range required per level.
	$age_required_per_level = array(
		// Level 3 requires a minimum age of 21 and a maximum age of 25
		3 => array(
			'min' => 21,
			'max' => 25,
		),
		// Level 4 requires a minimum age of 26 and a maximum age of 40
		4 => array(
			'min' => 26,
			'max' => 40,
		),
		// Level 5 requires a minimum age of 41 and a maximum age of 120
		5 => array(
			'min' => 41,
			'max' => 120,
		),
	);

	// Get the level.
	$level    = pmpro_getLevelAtCheckout();
	$level_id = $level->id;

	// Bail if things are not okay or if the level is not one that requires a birthday.
	if ( ! $okay || ! array_key_exists( $level_id, $age_required_per_level ) ) {
		return $okay;
	}

	// Get the birthday
	if ( isset( $_REQUEST['birthday'] ) && ! empty( $_REQUEST['birthday'] ) ) {
		$birthday = $_REQUEST['birthday'];
	} else {
		pmpro_setMessage( 'Please fill out the Birthday field.', 'pmpro_error' );
		return false;
	}

	// Check if the birthday values are numeric and is a valid date.
	if ( ! 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 birthday.', 'pmpro_error' );
		return false;
	}

	$now = new DateTime(); // Get the current date.
	$dob = new DateTime( $birthday['y'] . '-' . $birthday['m'] . '-' . $birthday['d'] ); // Get the date of birth.

	// Calculate the age of the user.
	$age = $dob->diff( $now )->y;

	// Check if the user is within the required age range for the level.
	if ( $age < $age_required_per_level[ $level_id ]['min'] || $age > $age_required_per_level[ $level_id ]['max'] ) {
		pmpro_setMessage( 'You must be between ' . $age_required_per_level[ $level_id ]['min'] . ' and ' . $age_required_per_level[ $level_id ]['max'] . ' years old to register for this level.', 'pmpro_error' );
		return false;
	}

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