Its pretty simple really. Kids to young to understand the permancy of death don't have a reason to react. Older kids and adults grieve according to closeness of the relationship, not title of the deceased. If my half-brother (whom I've never met) dies, I will feel likely little if any emotion. However, if my close friend of 3 years died, or God forbid, one of my pets, I'll be heartbroken. Additionally, we all grieve differently and even the kubler Ross stages don't just happen in a linear fashion. No one should ever judge themselves at how they handle grief.
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"Which one? Oh, the creepy one? Neat, I hated running into him".