Make any post older than 18 months available for free with Paid Memberships Pro.

make_old_posts_free.php

<?php

/**
 * This recipe allows non-members to view specific restricted posts
 * if they are older than the amount of days specified per post.
 *
 * To set which posts to unlock and their individual period (days)
 * set the post id's as the array key and the period as the
 * value of the array key of the $posts_to_unlock variable.
 *
 * 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 make_old_posts_free( $hasaccess, $post, $user, $post_membership_levels ) {
	//if they already have access, let them at it
	if ( $hasaccess ) {
		return $hasaccess;
	}

	//only make posts of type post (i.e. not pages or other CPTs) free
	if ( 'post' !== $post->post_type ) {
		return $hasaccess;
	}

	// Set your post id's and the unlock period here.
	$posts_to_unlock = array(
		36 => '-30 Days', // post ID 21, 30 days old
		35 => '-20 Days', // post ID 35, 20 days old
		41 => '-10 Days', // post ID 155, 10 days old
	);

	//now check the publish date
	$post_id   = $post->ID;
	$published = strtotime( $post->post_date, current_time( 'timestamp' ) );
	$open_date = strtotime( $posts_to_unlock[ $post_id ], current_time( 'timestamp' ) );

	if ( $published < $open_date && array_key_exists( $post_id, $posts_to_unlock ) ) {
		$hasaccess = true;
	}

	return $hasaccess;
}
add_filter( 'pmpro_has_membership_access_filter', 'make_old_posts_free', 10, 4 );