No age groups exists
$meta_query = array( 'relation' => 'AND', array( 'key' => 'age_group_to', 'compare' => 'NOT EXISTS', ), array( 'key' => 'age_group_to', 'compare' => 'NOT EXISTS', ), );
$meta_query = array( 'relation' => 'AND', array( 'key' => 'age_group_to', 'compare' => 'NOT EXISTS', ), array( 'key' => 'age_group_to', 'compare' => 'NOT EXISTS', ), );