Consider a group made up of five friends aged 13, 13, 14, 15 and 15 years old. What happens to the average age of this group if a sixth friend who is 8 years old joins the group?

The average of a group made up of 5 friends is 14 while the average age of the group if a sixth friend joins the group is 13. Thus, we can say that the average age of the group decreased.