Young children acquire the grammar of their native language to a fluent, effectively native-level standard within the first several years of life, and they do this without any explicit grammatical instruction at all, simply from ordinary conversational exposure to the language being spoken around them. This pace and completeness of language acquisition is remarkable enough on its own, but it becomes especially striking by comparison: adults deliberately, effortfully studying a second language later in life very rarely reach the same effortless, complete grammatical fluency a young child achieves purely through everyday exposure.
Children extract complex grammatical rules without ever being taught them explicitly
A young child acquiring their native language isn't given systematic grammar lessons the way a student studying a foreign language typically is; instead, they extract the underlying grammatical patterns and rules of their language purely from hearing it used around them in everyday conversation, gradually producing increasingly sophisticated, grammatically correct sentences without ever being explicitly told the specific rule governing, say, how to correctly form a past tense or arrange a complex sentence's clauses. This ability to extract complex, abstract grammatical structure from unstructured, everyday linguistic input, entirely without formal teaching, is one of language acquisition's most striking and well-documented features.
Adults learning a new language rarely match this speed and completeness
Adults setting out to learn a new language, even with considerably more deliberate study time, explicit grammar instruction and conscious motivation than a young child ever receives, very rarely achieve the same effortless, complete native-level grammatical fluency a young child reaches purely through ordinary exposure. This striking asymmetry, between children's apparently effortless acquisition and adults' typically effortful, often incomplete learning, is a central, well-documented feature of language acquisition research, and it's a major part of why researchers have proposed that some kind of especially favourable developmental window for language acquisition may exist earlier in life.
What we're still unsure about
That young children reliably acquire complex native-language grammar from ordinary exposure alone, without explicit instruction, and that adult second-language learners typically don't match this same speed and completeness, are well documented, extensively studied findings in developmental psycholinguistics. What remains genuinely, actively debated is exactly why this asymmetry exists, whether it reflects some specific biologically limited developmental window for language specifically, more general changes in overall learning capacity and flexibility that happen with age, or some combination of both — researchers continue to actively investigate and debate the precise underlying cause, without a single, fully settled explanation for why the acquisition gap between children and adults is quite as large and consistent as it is.
This sits inside Language Acquisition, one of seven topics in Developmental Psychology, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.