This code will create a content filter for pages to remove access to posts that were published before a member's join date, including a 15 day grace period.

studiotogo_hide_old_pages_from_members.php

<?php
/*
 * This code will create a content filter for pages to remove access to posts that were published before a member's join date.
 * It includes a 15 day "grace period" so that members can see posts published up to 15 days prior to their join date.
 *
 * The params passed are:
 * $hasaccess - (bool) what PMPro thinks about whether the user has access
 * $thepost - (post object) the post being checked, usually the current post
 * $theuser - (user object) the user being checked, usually the current user
 * $post_membership_levels - (array of levels) the levels this post requires (if any)
 */
function studiotogo_hide_old_pages_from_members($hasaccess, $thepost, $theuser, $post_membership_levels)
{
	global $wpdb;
	//if PMPro says false already, return false
	if ( ! $hasaccess ) {
		return false;
    }
    
    //if the post isn't a "page" just return.
	if ( $thepost->post_type != 'page' ) {
		return $hasaccess;
    }

	//if the post doesn't require membership, allow access
	if ( ! $post_membership_levels ) {
		return true;
    }

	//okay, this post requires membership. start by getting the user's startdate
	$startdate = pmpro_getMemberStartdate( $theuser->ID );
    $grace_startdate = strtotime( '-15 days', $startdate );
    
	//no startdate? return false
	if ( empty( $grace_startdate ) ) {
		return false;
    }
    
	//if the startdate is before the post date, return true
	if ( $grace_startdate < strtotime( $thepost->post_date, current_time( 'timestamp' ) ) ) {
		return true;
    } else {
		//in this case we want to also tweak the message shown
		add_filter( 'pmpro_non_member_text_filter', 'swap_old_posts_member_text' );
		return false;
	}
}
add_filter( 'pmpro_has_membership_access_filter', 'studiotogo_hide_old_pages_from_members', 10, 4);

// Swap the noaccess message.
function swap_old_posts_member_text( $s ) {
	$s = "This content was published before your membership started.";
	return $s;
}