Q. A six year old little boy asks: "Why do people die?"
Here is the Biblical answer:
Romans 5:12 – 19 (NASB’95 edition)
12 Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all men, because all sinned–
13 for until the Law sin was in the world, but sin is not imputed when there is no law.
14 Nevertheless death reigned from Adam until Moses, even over those who had not sinned in the likeness of the offense of Adam, who is a type of Him who was to come.
15 But the free gift is not like the transgression. For if by the transgression of the one the many died, much more did the grace of God and the gift by the grace of the one Man, Jesus Christ, abound to the many.
16 The gift is not like that which came through the one who sinned; for on the one hand the judgment arose from one transgression resulting in condemnation, but on the other hand the free gift arose from many transgressions resulting in justification.
17 For if by the transgression of the one, death reigned through the one, much more those who receive the abundance of grace and of the gift of righteousness will reign in life through the One, Jesus Christ.
18 So then as through one transgression there resulted condemnation to all men, even so through one act of righteousness there resulted justification of life to all men.
19 For as through the one man's disobedience the many were made sinners, even so through the obedience of the One the many will be made righteous.
It's a deep subject, but one that should be well understood by all people. Sin is the reason for physical death. Although we do not share the guilt of Adams' sin, we suffer the consequences because in reality, we all sinned.
I usually explain it in terms of probation. We were all on probation in the garden. Adam, the "representative" man did what we would have done, because "all have sinned and fallen short of the Glory of God" (Romans 3:23). And, "the wages of sin is death" (Rom. 6: 23). Therefore, physical death was imposed as a consequence of sin.
This helps us to see the serious nature of sin. It is not something to be taken lightly. It is serious. The good news is that Jesus Christ died to take away the guilt of my sin, and His death also gave me the opportunity for physical life (Romans 5:12-19) even though I did not deserve it!
Much too deep a subject for your little one, and probably too deep for most folks I know! The important lesson to learn is the serious nature of sin. Death reminds us that God takes a very dim view of disobedience.