You can also ask Clyde in his forum to look in the archives for you.

I'm not sure what exactly you mean by just "show up in this day and age"
Are you talking about it not happening until the 20th century, or not until someone is older that it's found?

Of course, we don't know how much of it was being suffered prior to mental health study (basically begun by Freud.)

You can google it and see that there are a few very early documented cases (sometimes called by various names.)

I think it has become more widely recognized (as a true disorder and as being found in people) because of the studies and work on PTSD after the Vietnam War. With that understanding continuing to grow, and with the issues that often the mechanism of DID begins to break down and not be as effective as someone ages, could be why we are noticing so much of it. (Estimations are from 1 in 100 to 1 in 500 or more have this disorder.)