**Core Concept**
The question is testing the understanding of the pediatric growth cycle and the transition from childhood to adolescence. This transition is marked by significant physical, emotional, and social changes. The growth cycle is divided into several stages, including infancy, childhood, and adolescence.

**Why the Correct Answer is Right**
The correct answer is 10 years because the World Health Organization (WHO) and various pediatric guidelines consider 10 years as the age when childhood ends and adolescence begins. This is based on the physical and emotional changes that occur during this period, such as the onset of puberty, growth spurts, and changes in body composition. At 10 years, children start to exhibit more adult-like characteristics, both physically and emotionally, marking the end of childhood and the beginning of adolescence.

**Why Each Wrong Option is Incorrect**
**Option A:** 8 years is too early for the transition from childhood to adolescence, as most children at this age are still in the early stages of development and have not yet reached puberty.

**Option C:** 12 years is also incorrect because while some children may start to exhibit early signs of puberty around this age, it is not universally accepted as the age when childhood ends and adolescence begins.

**Option D:** 16 years is incorrect because while this age is often associated with the end of adolescence and the beginning of young adulthood, it is not the age when childhood ends and adolescence starts.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the growth cycle and the transition from childhood to adolescence can vary significantly among individuals, and 10 years is only a general guideline. Pediatricians and healthcare providers use various assessments, such as growth charts and developmental screenings, to evaluate a child's overall development and identify any potential issues.

**✓ Correct Answer: B. 10 years**