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The critical period for language acquisition is when children's ability to learn language peaks early in life. Proficiency decreases with age, meaning younger children learn new languages more easily than adults. This biological window suggests that early exposure to language is optimal for developing native-like fluency and linguistic competence.

Newborns show a preference for their mother's voice and can discriminate between their mother's language and other languages. Initially, babies can differentiate among sounds of all human languages. However, by around one year old, they can only discriminate among phonemes used in their environment, showing how early exposure shapes language perception.

During the babbling stage, babies produce repeated single syllables, which may occur alone or with caregivers. This babbling is not necessarily communicative. Babies exposed to sign language will also babble with their hands, demonstrating that the babbling stage reflects a biological drive to produce language-like sounds regardless of modality.

Children typically utter their first word between one year and 18 months, entering the one-word stage where single words carry larger meanings. For example, 'cookie' might mean they want a cookie or are identifying one. Early vocabulary is often limited to familiar objects or events, primarily nouns that relate to their immediate experience.

As children's vocabulary grows, they begin to form simple sentences and rapidly acquire new words. They demonstrate understanding of language rules, sometimes making mistakes through overgeneralization, such as saying 'gooses' instead of 'geese.' These errors show children are actively learning grammatical patterns and applying them systematically.

B.F. Skinner proposed that language is learned through reinforcement, emphasizing environmental factors. Noam Chomsky argued that language acquisition mechanisms are biologically determined, suggesting an innate capacity for language. This debate highlights the nature-versus-nurture question in how children master language quickly with both biological predisposition and environmental support.

Language development follows consistent stages: reflexive communication emerges at 0-3 months, intentional communication develops by 8-13 months, first words appear at 12-18 months, two-word sentences form at 18-24 months, and complex sentences develop by 2-3 years. By 3-5 years, children engage in sophisticated conversations with improved grammar and syntax understanding.