**Core Concept**
Language development in children is a complex process influenced by both genetic and environmental factors. The emergence of sentence formation is a significant milestone in language acquisition, reflecting improvements in cognitive and linguistic abilities.

**Why the Correct Answer is Right**
Typically, children begin speaking single words around 12-18 months of age. However, sentence formation, which involves combining multiple words to convey meaning, usually develops around 2 years of age. This is because children need time to develop their understanding of grammar, syntax, and semantics, as well as to refine their motor skills for articulation. The emergence of sentence formation is often accompanied by improvements in vocabulary and the ability to engage in more complex conversations.

**Why Each Wrong Option is Incorrect**
**Option A:** 6 months is too early for sentence formation, as children at this age are primarily focused on developing their auditory skills and understanding simple sounds and tones.
**Option B:** 1 year is also too early, as children at this age are typically still using single words to communicate and have not yet developed the cognitive skills necessary for sentence formation.
**Option C:** 18 months is close, but sentence formation usually occurs after this age. Children at 18 months may be able to combine two words together, but they have not yet developed the ability to form complex sentences.

**Clinical Pearl / High-Yield Fact**
Keep in mind that language development can vary significantly among children, and some may develop sentence formation earlier or later than their peers. However, as a general rule, sentence formation typically emerges around 2 years of age.

✓ Correct Answer: D. 2 years