Final answer:
The pronunciation of 'r' is typically fully developed by age five, when children's speech and language skills have matured significantly.
Step-by-step explanation:
The pronunciation of 'r' is usually fully developed by age five. At this age, children have a vocabulary of at least 1,500 words, are able to produce sentences of at least five to seven words, and can define words by their function. They experience significant language development during the early childhood years, marked by a rapid increase in vocabulary and the ability to engage in complex communication. Although the ability to pronounce certain phonemes like 'r' may vary among individual children, age five is generally when you can expect this aspect of speech to be well established.