Можно так:
$age = \DateTime::createFromFormat(self::$sqlDtFormat, $item['birth_date'])
->diff(new DateTime('now'))
->y;
Или с установкой временной зоны:
$tz = new DateTimeZone('Europe/Brussels');
$age = DateTime::createFromFormat('d/m/Y', '12/02/1973', $tz)
->diff(new DateTime('now', $tz))
->y;