Python and Dart, for example, have been much more popular recently than in the past, so it's reasonable to assume that the percentage of newer code bases in those languages is higher than in, say, Java. Python may have been popular already 10 years ago, but it feels like back then it was mostly used for small websites and scripting, things you don't normally need to maintain for a long time.
> So quite trivially it's not that the languages are too new to have old crusty codebase ...
Considering the year a language was created is missing the point the author tried to make, unfortunately, so I would say your argument is based on a flawed premise (trivially shown by reading the article and understanding it).
I think JS is the only language that goes against this, but that's quite easy to explain: it's a language used mostly for web development, where it's practically, your only choice (notice that "loved" actually means having intent to continue using it in the future).