Children progress through distinct stages in acquiring their first language from birth to age 4: (1) Early vocalizations and babbling reflect the sounds of the language being learned. (2) Around 12 months, children say their first words. (3) By age 2, children start combining words into simple sentences of 2 words. (4) By age 4, children are able to use complex language structures like questions and commands most of the time. Theories on how children acquire their first language include behaviorism through imitation and reinforcement, as well as nativism which posits an innate language acquisition device.