What is a good age to teach children about death?

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My son is 6 years old and has know about his father being deceased since he was born. Recently my grandmother passed away and I took my children to her funeral. My daughters father knew I was taking the kids and that they would be there. He said nothing about not wanting our daughter to be there until after everything was done. I talked to him before the funeral and while we were at the funeral. Our daughter is four years old and never went near the body she was in a separate room with all the other children. What is a good age to teach a child about death? What is a good age to take a child to a funeral?