Meta NOT EXISTS

$meta_query = array(
	'relation' => 'OR',
	array(
		'relation' => 'OR',
		array(
			'relation' => 'OR',
			array(
				'relation' => 'AND',
				array(
					'key'     => 'age_min',
					'compare' => 'NOT EXISTS',
				),
				array(
					'key'     => 'age_max',
					'compare' => 'NOT EXISTS',
				),
			),
			array(
				'key'     => 'age_from',
				'value'   => 'foobar',
				'compare' => '=',
			),
			array(
				'key'     => 'age_to',
				'value'   => 'foobar',
				'compare' => '=',
			),
		),
		array(
			'relation' => 'AND',
			array(
				'key'     => 'age_from',
				'value'   => '$ageMin',
				'compare' => '<=',
			),
			array(
				'key'     => 'age_to',
				'value'   => '$ageMax',
				'compare' => '>=',
			),
		),
	),
	array(
		'relation' => 'AND',
		array(
			'key'     => 'age_from',
			'value'   => '$ageMin',
			'compare' => '<=',
		),
		array(
			'key'     => 'age_to',
			'value'   => '$ageMax',
			'compare' => '>=',
		),
	),
);