Final answer:
Teenagers are defined as individuals between the ages of 13 and 19. The age of 18 is when one typically becomes a legal adult but is still regarded as a teenage year.
Step-by-step explanation:
The age limit that defines teenagers is the range of years when a person is between the ages of 13 and 19. This period corresponds to the teen numbers in age and covers the stage of life from the beginning of adolescence to the brink of adulthood. While reaching the age of 18 is commonly associated with legally becoming an adult, it does not change the fact that 18 and 19 are still considered teenage years. Therefore, the correct answer to what age limit defines teenagers is b. 18.