The Individual Division automatically includes all men and women between 16 - 54 years old. Athletes ages 14 - 17 are placed in the Teenage Division, and athletes ages 35+ are placed in the Masters Division.
As you may have noticed, there's overlap between the Individual and Masters Divisions (35 - 54) and the Individual and Teenage Division (16-17). The youngest four masters age groups (35-39, 40-44, 45-49, 50-54) and Teenagers 16-17 will always complete the same version of the Open workout as the Individual Division, and therefore these athletes can appear on both leaderboards (Individual and Masters or Individuals and Teenagers).
Should they finish the Open ranked high enough on the Individual Leaderboard they are eligible to compete at the Games against Individual athletes.