wordpress ajax call, load more posts

ajax.php

// chiamata ajax per la pagina category load more posts 
function load_old_post()
{
    $offset = $_GET['offset'];
    $category = $_GET['category'];
    $args = array( 'posts_per_page' => 2 , 'cat' => $category, 'offset' => $offset );
    $theme_post_query = new WP_Query( $args );    
    while( $theme_post_query->have_posts() ) : $theme_post_query->the_post();?>
        <?php $category = get_the_category(); $category = $category[0]; ?>
        <?php $parent_cat = get_cat_name( $category->parent ); ?>
        <?php  
        $fields = get_fields();
        $luogo = $fields['luogo'];
        $url = get_the_permalink();   
        ?>
        <article class="<?php  if($parent_cat != NULL) : echo strtolower($parent_cat); else : echo $category->slug; endif;  ?> cf">
            <a href="<?php echo the_permalink(); ?>">
                <!-- post thumbnail -->
                <?php if ( has_post_thumbnail()) : // Check if thumbnail exists ?>
                    <figure><?php the_post_thumbnail(array(900,600));?></figure>
                <?php else : ?> <figure><img src="<?php echo get_template_directory_uri(); ?>/img/surfer.jpg" alt="" title="" /></figure>
                <?php endif; ?>
                <!-- /post thumbnail -->
                <div class="date"><span><?php echo $luogo; ?></span> &#126; <?php echo the_time('d • m • Y'); ?></div>
                <h2><?php echo get_the_title(); ?></h2>
            </a>
            <hr />
            <div class="description"><?php echo wp_trim_words(get_the_excerpt(),20 ); ?></div>
            <footer class="article-social cf">
                <a target="_blank" href="<?php echo fbShare(get_the_permalink(),wp_get_attachment_url( get_post_thumbnail_id())); ?>" class="facebook-link"><span>facebook</span></a>
                <a target="_blank" href="<?php echo twitterShare(get_the_permalink()); ?>" class="twitter-link "><span>twitter</span></a>
                <a target="_blank" href="<?php echo pinterestShare(get_the_permalink(),$thumbnail_url,$title); ?>" class="pinterest-link"><span>pinterest</span></a>
            </footer>
        </article>

    <?php endwhile;         
    exit;
}


add_action('wp_ajax_load_old_post', 'load_old_post');
add_action('wp_ajax_nopriv_load_old_post', 'load_old_post');