My experience is that there's really no lower limit, the only thing that matters is to sort of keeping it on level that the child can comprehend. With that I mean that it is important to talk about "death" as "death" and not through far fetched metaphors.
Through my job I've worked with kids ranging from months, through a few years and up that have taken part of death - either visiting a dying relative or following when the dead is taken farewell of. The real problem is not the kid, but all grown ups that have their own macabre ideas on death, and that more often than not in some misleading way want to protect the kid from The Unmentionable (where "detah" as a phenomena seems to be very problematic, and therefore Unmentionable, for many adults).
A Norwegian professor in clinical psychology has done some quite extensive writing on the subject of small children, grief and death. His name is Atle Dyregrov and he works at the University of Bergen. Beside science he's been involved in training teachers and parents, as well as guiding schools etc, in case of emergencies and deaths. His writings are published in English if I remember correctly, and he's quite easy to read.