Final answer:
An infant can understand a few words and have partially intelligible speech by the age of one year, with continued rapid language development occurring until the age of three.
Step-by-step explanation:
An infant generally understands a few words and begins to have partially intelligible speech around the age of one year. By 12 months, infants may say their first word, often a compelling 'mama' or 'dada', and are able to understand simple commands, particularly when accompanied by gestures. By the end of the first year, the infant's babbling begins to resemble speech patterns more closely and may demonstrate understanding of basic words beyond their few spoken ones. This understanding and expression improve significantly as the toddler approaches two years old, where their vocabulary can grow to include as many as 50 spoken words. Language acquisition is a rapid process between nine months and three years, with the child's vocabulary increasing quickly during this span.
By the time the child is two years old, they not only speak around 50 words but also understand at least a couple hundred more, showing even greater communicative skills. They continue to develop cognitive abilities related to language and improve their ability to follow commands and engage in problem-solving activities.