How Do You Know Where the Words Start and Stop?

When you hit a certain age, maybe 13 for the sound of a language, 17 for the syntax, it shuts down mostly. And second language learning is murder. So gradually, starting off English sounds to a kid like Chinese, or Hebrew sounds to me just as mishmash and noise. We can't even understand the idea that somebody couldn't hear it as much as this thing. Works that our mind puts in the slots.