**Core Concept**
Children typically develop an understanding of their own sex between 2 to 4 years of age. This is a critical milestone in cognitive development, often associated with language skills and self-awareness.

**Why the Correct Answer is Right**
The correct answer, 3 years, is based on Piaget's theory of cognitive development. Children at this age begin to understand the concept of their own sex through social interactions and language. They start to identify themselves as boys or girls and begin to understand the differences between the two sexes. This understanding is a result of the development of their egocentrism, which allows them to think about themselves and their place in the world.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 year is too early for children to understand their own sex. At this age, they are still developing their basic cognitive skills and have not yet reached the stage of egocentrism.

**Option C:** 5 years is too late for children to have already developed an understanding of their own sex. While some children may take longer than others, most children have a basic understanding of their sex by around 3 years of age.

**Option D:** 7 years is not relevant to the typical age range for children to develop an understanding of their own sex. While some children may continue to develop their understanding of sex and gender at this age, it is generally not a milestone that is achieved at 7 years of age.

**Clinical Pearl / High-Yield Fact**
It's essential for pediatricians to be aware of the normal developmental milestones, including the understanding of one's own sex, to identify any potential developmental delays or disorders.

**✓ Correct Answer: B. 3 years**