Health

both the apparent lack of infection and serious side effects in children, and the extremely high mortality rate among older people, is all tied to their immune systems. Children before puberty are still strengthening their immune systems which run at a high rate all the time.
Once you hit about 50 years old, your immune system starts to decline.

This all reminds me of a comment a professor made in a college biology course. She said to keep in mind that thousands of years of evolution had designed us to live only until we were in our late 30s. By then, women were not bearing children and men were probably too broken down by injury and disease to be helpful to the tribe as hunters. A non-productive person in a small group that has a daily struggle to get enough to eat and survive is a liability, so life after 40 was not a trait that was adapted by natural selection.