So, I think the idea runs like this: "If it happened 1000 years ago and someone still cares enough about it to write an article
then it's
probably notable enough to keep."
Interestingly, people /will/ try and delete something if the subject is only 100-200 years old, but it's perfectly acceptable to have articles on every X in Y where Y is 1000 times larger that all other Z, where Y is a member of Z, and yet not allowed to have articles on other X's in W, where W is also a member of Z, and W is 10000 times smaller than Y...
Interesting. Can anyone else vouch for this? It sounds like another principle that shapes people's ideas on notability: Articles about older subjects have more right to appear in Wikipedia than articles about newer subjects.
Steve