Let me ask you the following: When learning a language, with which version would you begin learning yourself? We are at C++14, C11, Object-Pascal 10.1, JS ES6, HTML5.1, Perl6, etc. If you would pick up one of those without any knowledge, would you start by learning an old version? What advantages would you see in learning an old version?
Sure, the old features might still work, but people actually put a lot a thought into the new features. They exist for a reason. Why should anyone learn the tedious way of having default parameter values in JS ES5, when we have destructuring?
Yes, the old versions are closer to the "basics", but let's think ahead a bit... when ES10 comes out. Would anyone still care about ES5? Would anyone explain JS without explaining things which became available in ES7? Why don't you think about teaching JS 1.0 or ES3? Why ES5? There are a lot of features which are "difficult to understand" (not really) for beginners which were introduced in ES5!
ES6 is not some kind of new thing. It's just the next version of JS as it will be used now and in the future. It's not harder to learn. The only thing making it harder is your mindset, which says that that stuff is new. For a beginner, however, everything is new! ES6 will be regular stuff, and when ES7 is released, that will be the first new and changed stuff for a beginner who learns JS now.
Do not make the mistake of confusing a beginner with 0 knowledge and you with knowledge of the history and legacy ways to handle certain problems.
To put it into a nutshell: Teach the latest and the greatest.