<?php
include('Conexion.php');
$pdoData = array_filter((function($p) {
    $rut = $nombre = $edad = $fechaNacimiento = $nacionalidad =
    $sexo = $profesion = '';
    extract($p, EXTR_IF_EXISTS);
    return [
        'rut'=>$rut, 'nombre'=>$nombre, 'edad'=>$edad, 'profesion'=> $profesion,
        'sexo'=> ($sexo == 'male') ? 1 : 0, 'nacionalidad'=>$nacionalidad,
        'fechaNacimiento'=> date('Y-m-d', strtotime($fechaNacimiento)),
    ];
})($_POST), function($v) { return strlen($v);});
$required = array_flip(['rut', 'nombre', 'edad', 'nacionalidad', 'fechaNacimiento']);
if (count(array_diff_key($pdoData, $required))) {
    // a required field is missing; stop processing here and do something
    // reasonable to let the user fix it; note that "die()" is not a good
    // example of "something reasonable"; I include it here only to show
    // that the expectation of the following code is that all required
    // data is available.
    die("Required fields missing");
}

try {
    $sql = 'INSERT INTO usuario(rut, nombre, edad, fechaNacimiento, sexo, nacionalidad, id_profesion)
                  values(:rut, :nombre, :edad, :fechaNacimiento, :sexo, :nacionalidad, :id_profesion)';

    $stmt = $pdo->prepare($sql);
    $stmt->execute($pdoData);
} catch (\PDOException $e) {
    // again, you need to do something REASONABLE here; don't just vomit out
    // errors - actually show useful data to the end user about what went
    // wrong and help them fix it.
}