"around what age are most children fully capable of engaging in fluent conversation with speakers of any age? '

By age 8 or 9, children change to a new level of accepting of the structure of language, figuring out relations between whole groups of words, for example between adjectives and adverbs or between adjectives and nouns. Knowing and understanding these relationships, the child can now comprehend and make a whole group of new words and his or her language grows quickly. So by age 9, most of the children are completely proficient of engaging in easy conversation with speakers of any time, and their speech rates method those of adults.