2020 feels like a new age because of the browser ecosystem. The actual endpoint of the "second age" is probably the combined forces of:

1) Windows 7 Support officially ended

2) Edge now switched over to Chromium

With the exception of highly niche applications and markets, sites meant to be consumed on computers can aim for Chromium support and developers can rest assured they are covering the overwhelming majority of the market. Many slower-moving enterprises are standardizing on Chromium-powered browsers. Even NetFront (browsers used by video game consoles) is switching over to Chromium

The "Third Age of JavaScript" isn't marked by tooling or frameworks or code structure, but rather by developers able to focus on one browser instead of multiple. This new age will be marked by innovative use of APIs that people disregarded in the past because they only worked in Chrome.