Set a max age for WP pages

wp-cache-controller.php

<?php
/**
 * Plugin Name: Cache Controller
 * Plugin URI:  #
 * Description: HTTP Cache Control headers for WordPress.
 * Version:     0.0.1
 * Author:      James Morrison
 * Author URI:  https://james.morrison.me/
 **/

// Security check
defined( 'ABSPATH' ) || die( 'Direct file access is forbidden.' );

// Send headers
add_action( 'template_redirect', function() {

    if ( is_user_logged_in() || defined( 'LOCAL_DEV' ) && LOCAL_DEV || defined( 'WP_LOCAL_DEV' ) && WP_LOCAL_DEV ) {

        header( "Cache-Control: no-store, no-cache, must-revalidate" );
        header( "Cache-Control: post-check=0, pre-check=0", false );
        header( "Expires: Fri, 30 Dec 1983 07:30:00 GMT" );
        header( "Pragma: no-cache" );
        header( "Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT" );

    } else {

        header( "Cache-Control: public, max-age=300" );

    }

} );