[tkgally]

The usual explanation is that younger people, especially small children, can pick up languages without explicit knowledge of the grammar better than older people can. There's some controversy about whether there is a narrow age range after which it becomes difficult or impossible to acquire native-level proficiency through immersion or whether the decline in that ability is gradual [0]. But there's a pretty strong consensus that there is a decline for almost everyone.

While adults can learn second languages, very few can just pick up the grammar as they did with their native languages as children. For most adults, if we want to learn how to speak and write a second language with reasonable accuracy, we have to study the grammar explicitly. Practicing in immersive situations also helps a lot, of course.

[0] https://en.wikipedia.org/wiki/Critical_period_hypothesis