I don't think Bernie runs in 2020. Not only will he be very old, but he'd not really do as well as 2016.
He was a "I don't like Hillary, so I'm voting for him, whoever this guy is" vote for a lot of people (check out WV), and at this point, he can't afford to decrease his base any further. He lost, so just his 2016 base, copy and pasted to 2020 wouldn't work. "Establishment" Democrats will always outnumber Bernie voters, so unless he magically knows how to appeal to minorities (he doesn't) and establishment Democrats (also doesn't), he's not going to get even close to his 2016 levels, and not a prayer of actually winning.
He was at the right time to get the support he did. An extremely unliked "sure winner" in a field with absolutely no competition at all. With 2020 likely being a crowded field, and no front runner right out of the gate, where is Bernie's niche?