**Core Concept**
The term "adolescence" refers to a stage of human development that spans from puberty to adulthood, characterized by significant physical, emotional, and psychological changes. This period typically begins and ends at specific ages, with the World Health Organization (WHO) defining adolescence as the period between 10 and 19 years of age.

**Why the Correct Answer is Right**
The correct answer is based on the WHO's definition of adolescence, which emphasizes the age range of 10 to 19 years. This age range is considered critical for development, as it marks the transition from childhood to adulthood, during which individuals experience rapid physical growth, changes in body composition, and significant psychological and emotional development. The onset of puberty, which typically begins around 10-11 years of age, marks the beginning of this stage.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it marks the beginning of early childhood, which is a distinct stage of human development that precedes adolescence.
**Option B:** This option is incorrect because it marks the beginning of young adulthood, which is a stage that follows adolescence.
**Option C:** This option is incorrect because it marks the end of adolescence and the beginning of young adulthood.

**Clinical Pearl / High-Yield Fact**
A key aspect of adolescence is the rapid physical growth and development that occurs during this stage. Specifically, the adolescent growth spurt, which typically begins around 10-11 years of age, is characterized by rapid increases in height and weight, as well as changes in body composition.

**Correct Answer: C. 13. Adolescence starts at the age of 13.