The title of a video of Bafut music recorded in Nigeria, the Delta. It reflects what can happen when a language is transmitted incompletely as, e.g. happened when Vikings learned English but incompletely. Many changes in our grammar occurred due to that and went on to become part of our language, some of the standard language and some of non-standard varieties.
Here, the use of “year” without an -s, the standard, goes against the expected use of the plural with 600. We know that in earlier English the number was followed by the word ‘year’ in a particular case which dropped off long ago, leaving the bare stem which also serves as the singular form. So it appears that ‘year’ is a singular after 600 where plural ‘years’ would be expected. And sure enough, we have a non-native speaker using the expected plural.
Such subtle changes occur in a language when large numbers of non-native speaker learn it and, in the process, simplify it. Who has time to decided that it is standard to say, “Six hundred years ago….” but “A six hundred year old culture….”?
The same thing happened to Arabic, Persian, Mandarin, and many other languages. See John McWhorter.