Unless the Merchant himself is some kind of immortal - which he very well could be - the emergence of these undead appears to not be extremely old. Older than a single man's life, but still in the realm of a story that could have been passed down.

So far, Anurr is the oldest one we have met, and he's probably 2 or 3 centuries old at the most. The other ones are younger. Young enough that some people alive remember life before them, or before they were turned.