Final answer:
Children are generally expected to be fully intelligible to strangers by the age of five, which includes having a developed vocabulary and the ability to construct sentences.
Step-by-step explanation:
Children should be fully intelligible to strangers by five years of age. By this age, kids have developed a communication skill set that includes a significant vocabulary, the ability to create sentences with multiple words, and the capacity to define words by their function. They are also sociable, can recount simple jokes, and are capable of more complex thought processes like sorting objects by multiple attributes and understanding the concepts of calendars and clocks.
Language development is a significant part of early childhood, and by the time children reach five, their speech and linguistic abilities have typically advanced to a level where strangers can understand them. It's noteworthy that prior to this age, as toddlers and young children, they undergo substantial cognitive and linguistic growth. By the age of three, kids can speak simple sentences and ask questions, and one-year-olds start using single words and follow simple commands. By the time they are six months, infants begin to make two-syllable sounds such as "mama" or "dada", and soon after that, they experience rapid language learning that continues until around three years of age.