A couple of hundred years ago, though Darzem was never really at home to notice. Ever since her third host Jorzon died young in an engineering accident - the Knox symbiont felt too guilty leaving behind his wife and son, never remarried or had anymore children. None that Nazar knows of in any case!
Sirzi would have been the best, but he was their first host 300 odd years ago. Children have changed a little since then!