Marie Nagy, in her article "The Child’s Theories Concerning Death," defines three stages of the understanding of death for children. Nagy broke her stages into age brackets: three to five, six to nine, and nine and above.
Three to Five
Children in this age bracket believe death to be reversible and temporary. Children tend to look at death as if the person is sleeping.
Six to Nine
While the understanding of death is more advanced than that of younger children, children of this age tended to personify death (think of the images of the Grim Reaper in movies). Death lives and breathes and literally takes people.
After the age of nine, children understand about death and the laws which accompany death.