$birthday = '1985-04-15'; //Дата рождения

$age = floor( ( time() - strtotime($birthday) ) / (60 * 60 * 24 * 365.25) ); //Формула

echo $age; //27

function getAge($birthdate, $ageTime = null/*now by default*/){
        return (new DateTime())
                ->setTimestamp($ageTime ? $ageTime : time())
                ->diff(new DateTime($birthdate))->format('%Y');
}