Death is fundamental to understanding life. These concepts need to be ingrained, the earlier the better.
As soon as they can grasp the concept, throw it at them.
-Maybe even earlier. See ramban in parshas nitzavim by mitzvah of hakel. Basically explains that kids come at a certain age even though they cannot fully understand now, because in the future the will have יראת שמים because they attended.
Also see chaggiga 3a.