Raw Source
nicolaslegland / agecheck 

// ==UserScript==
// @author      Nicolas Le Gland
// @copyright   2018-2019, nicolaslegland (https://openuserjs.org/users/nicolaslegland)
// @description Steam age check. Skip store birth date confirmation screen, as this data is for verification purposes only and will not be stored.
// @downloadURL https://openuserjs.org/src/scripts/nicolaslegland/agecheck.user.js
// @grant       none
// @homepageURL https://openuserjs.org/scripts/nicolaslegland/agecheck
// @icon        https://store.steampowered.com/favicon.ico
// @include     http://store.steampowered.com/agecheck/*
// @include     https://store.steampowered.com/agecheck/*
// @installURL  https://openuserjs.org/src/scripts/nicolaslegland/agecheck.user.js
// @license     GPL-3.0-or-later
// @name        agecheck
// @run-at      document-end
// @supportURL  https://openuserjs.org/scripts/nicolaslegland/agecheck/issues
// @updateURL   https://openuserjs.org/src/scripts/nicolaslegland/agecheck.user.js
// @version     10
// ==/UserScript==

// Set year
document.getElementById('ageYear').value = '1970';

// Click button
document.getElementById('app_agegate').getElementsByTagName('a')[0].click();