**Core Concept:** Thelarche is the early onset of secondary sexual development in girls, characterized by the development of breast tissue. It is a normal part of puberty and serves as an indicator of normal hormonal milestones in a child's development.

**Why the Correct Answer is Right:** Thelarche typically occurs in girls between the ages of 6 and 10 years, with a median age of around 8 years. This developmental stage is influenced by the rise in estrogen levels, which stimulates the growth of breast buds. As estrogen levels continue to rise, these buds expand and eventually mature into visible breast tissue.

**Why Each Wrong Option is Incorrect:**
A. 7 years is considered young for thelarche to occur, as the median age is typically around 8 years.
B. This option is incorrect because breast development does not usually occur in girls before the age of six years.
C. Although thelarche can occur in girls as early as 6 years, this option is too specific and does not encompass the broader range of ages (6-10 years) where thelarche is expected to occur.
D. This option is incorrect because breast development in boys (puberty) involves the development of male sex characteristics, not breast tissue in girls.

**Why Option A is Right:**
Option A (between 6 and 10 years) is correct because thelarche typically occurs within this age range, representing the normal developmental stage for breast tissue development in girls.

**Clinical Pearl:** A detailed history and physical examination are essential in evaluating children with developmental concerns. In addition to thelarche, other developmental milestones, such as the onset of menses, growth spurts, and voice deepening, can provide valuable information regarding a child's overall growth and development.

**Correct Answer:** Between 6 and 10 years, as thelarche occurs in girls between these ages, representing a normal part of puberty development.