How to perform a sql search on a custumer custom field?

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SELECT TIMESTAMPDIFF(YEAR, DATE(SUBSTR('{"1":"1977-04-07","2":"1"}',7,10)), DATE(NOW()))

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SELECT * FROM customer WHERE TIMESTAMPDIFF(YEAR, DATE(SUBSTR(custom_field,7,10)), DATE(NOW())) BETWEEN 35 AND 40

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"SELECT * FROM " . DB_PREFIX . "customer WHERE TIMESTAMPDIFF(YEAR, DATE(SUBSTR(custom_field,7,10)), DATE(NOW())) BETWEEN " . (int)$filter_age_min . " AND "  . (int)$filter_age_max