I'm playing a 100 baby challenge, and my matriarch's eldest children are now starting to die of old age. A few others died previously in accidents. I can find no rhyme or reason so far as to whether or not an urn is generated when they die.
As I mentioned, a few died in accidents. I had no problem finding urns on their lots.
A few more have now died of old age. I was able to find some of their urns, but not all of them. In the two cases where I could not find an urn, my sim had previously travelled to one of the lots but not the other. So, it doesn't seem to have anything to do with whether you've been to the lot before. One thing to note is that the two urns I could not find were for twin brothers who died of old age on different lots minutes apart from each other. That seems to be the only thing about their death that's unique from the others: twins dying of old age minutes apart. (There was another set of twins where one died in an accident; found his urn no problem.)
My sim has other sets of twins, but it'll be a while yet before they die of old age. I'll be watching to see whether the same thing happens with them. Regardless, I'm sad I won't be able to collect every one of my sim's children's urns. The family graveyard will never be complete...
