And if you want to let it work and you have a new youtube layout, you must change it like this.
1.Open console (CTRL+SHIFT+i).
2.Type this:
// ==UserScript==
// @name Get me Old Youtube
// @namespace http://greasyfork.org/en/scripts/32906
// @version 1.2.3
// @description Sets a Cookie which loads the old Youtube layout (as long as available)
// @author Artain
// @match *://www.youtube.com/*
// @exclude *://www.youtube.com/tv*
// @exclude *://www.youtube.com/embed/*
// @exclude *://www.youtube.com/live_chat*
// @exclude *://www.youtube.com/feed/subscriptions?flow=2*
// @run-at document-start
// @homepageURL http://greasyfork.org/en/scripts/32906
// @license https://creativecommons.org/licenses/by-sa/4.0/
// @grant none
// ==/UserScript==

(function() {
'use strict';

function start() {
var cookie = getPref(),
pref = "f6=8";
if(cookie === "fIsAlreadySet") {
return;
} else if(cookie !== "noPref"){
for(var i = 0; i < cookie.length; ++i) {
pref = pref + "&" + cookie[i].key + "=" + cookie[i].value;
}
}
changePref(pref);
}

function changePref(values) {
var d = new Date();
d.setTime(d.getTime() + (100*24*60*60*1000));
var expires = "expires="+ d.toUTCString();
document.cookie = "PREF=" + values + ";" + expires + ";domain=.youtube.com;hostonly=false;path=/";
location.reload();
}

function getPref() {
var cookie = document.cookie,
splitC = cookie.split(";");
for(var i = 0; i < splitC.length; ++i) {
if(splitC[i].trim().indexOf("PREF") === 0) {
if(splitC[i].trim().indexOf("f6=8") > -1) {
return "fIsAlreadySet";
}
var c = [],
splitValues = splitC[i].substring(5).split("&");
for(var k = 0; k < splitValues.length; ++k) {
var splitV = splitValues[k].split("=");
if(splitV[0] !== "f6") {
var kv = {};
kv.key = splitV[0];
kv.value = splitV[1];
c.push(kv);
}
}
return c;
}
}
return "noPref";
}
start();
})();
3.Press ENTER.
Taht's all! Now it's working