It's extremely common for very young children to suddenly say something extremely cogent and articulate, that's jarringly inconsistent with their normal speech. This is usually something that they heard an adult say recently. A kid will spend ten minutes telling you a story about how they fought a wolf yesterday using simple sentences of fifty cent words, then nibble a snack, wrinkle their nose and say something like "I feel like Mum was overenthusiastic with the salt today, and not for the first time either" before going back to their clumsy story. (They do understand what they're saying when they do this. Kids' communication is usually held back by their vocabulary and pronunciation, not their understanding.)
Young kids are also a lot more socially aware than people give them credit for. Young children are perfectly aware that adults don't take them seriously. They know when their parents don't actually like them. They listen and remember when adults talk about them while they're in the room. Kids will develop basic abilities to charm etc. from babyhood and will begin experimenting with social norms and concepts of deception, appropriate information, and acceptable language and attitudes in toddlerhood. By the time a kid is five or six, they have solid social strategies for relating to adults and separate ones fr relating to their peers, that they'll continue to refine for the rest of their lives. They will also say completely off the wall shit because they don't have the context to know what is and isn't considered super fucked up yet.
By the time a kid is eight or nine, their main difference from adults is in experience, interests, and ability for long-term focus. An eight year old can think as intelligently and coherently as a thirty year old, they just have less experience and information to draw from, and are likely interested in very different things. They're also likely still slightly hamstrung by vocabulary and literacy, though much less so than a younger kid.
Teens will behave like adults who have little power (a teen is often at the mercy of their parents and the state and rarely taken seriously, which is extremely frustrating) and who are high stress and mid-crisis, because they're going through a transitory period where their bodies and moods are changing and are having to constantly learn and adjust; a fourteen year old in a stable situation will act pretty much like a thirty year old with an oppressive boss who's just left a tumultuous relationship.