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' => '>=',
),
),
);