By the end of early adulthood, the most common reason for death is

It is "disease".

There are multiple reasons for getting sick in early adulthood. When an individual is young and thriving people normally tend minimal care for the health and well-being of themselves but being young their defensive system and body mechanism is capable of coping with most of the bacteria and disease generating factors, as one enters the early adulthood these situations become inverse and disease becomes the primary reason for the deaths.