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

make_old_posts_free.php

/*
	Make any post older than 18 months available for free.
	
	Add this code to your active theme's functions.php or a
	custom plugin.
*/
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_type != 'post')
		return $hasaccess;
		
	//now check the publish date
	$published = strtotime($post->post_date, current_time('timestamp'));
	if($published < strtotime("-18 Months", current_time('timestamp')))
		$hasaccess = true;

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

This recipe is included in the blog post on "Lock or Unlock Posts Based on Age and Post Date" at Paid Memberships Pro here: https://www.paidmembershipspro.com/lock-unlock-posts-based-age-post-date/