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event age

// WP_Query arguments
$args = array(
	'post_type'              => array( 'event' ),
	'meta_query'             => array(
		'relation' => 'AND',
		array(
			'key'     => 'age_min',
			'compare' => 'NOT EXISTS',
		),
		array(
			'key'     => 'age_max',
			'compare' => 'NOT EXISTS',
		),
	),
);

// The Query
$query = new WP_Query( $args );