Is the age of Count Janus Hassildor known? In what era was he born and turned into a vampire? In-game characters says he is centuries old. Is anything else known about him?

I don't recall anybody saying he was centuries old. I remember somebody saying he was infected about fifty years ago, and somebody else mentioning that he was "very public when my[their] grandmother was young" or something to that effect. I'd estimate from these two that he was approximately one hundred years old.

I wish I could remember the who the sources were though. It annoys me that I can't remember.

Yes, he was infected fifty years ago. I think he may have been about thirty to forty years old when he was infected though.