Unless it's completely necessary, I prefer not to put down dates, even rough ones. The reason isn't laziness on my part - I want to keep a good amount of them under wraps, and one of those things that I do want to keep under wraps, is how long they've been around nearby. A date appears non-vital to me in terms of tech, but story wise, is very important. For an example; even when players start interacting with them, talking or interrogating them, who's to say they're telling the truth of when they got or did this or that? I don't want hard numbers down in the Wiki quite yet so that with them, there can be a bit of a 'game' going on between them and the players when they're not even fighting.
Was it really 10 years ago? Or a hundred? Was it five? Or less and much more recently? It's that intrigue resulting from the lack of knowledge about them, and their willingness to 'mess with' the players by deliberately using it against them that I hope will make them into something everyone will absolutely love to hate.