All age group conditions
$meta_query = array( 'relation' => 'OR', array( array( 'key' => 'age_group_from', 'value' => '$ageMin', 'compare' => '<=', ), ), array( array( 'key' => 'age_group_to', 'value' => '$ageMax', 'compare' => '>=', ), ), array( 'relation' => 'AND', array( 'key' => 'age_group_to', 'compare' => 'NOT EXISTS', ), array( 'key' => 'age_group_to', 'compare' => 'NOT EXISTS', ), ), );