This is actually a somewhat controversial claim, academically. There's pretty strong evidence for a "critical period" for acquiring a native-sounding accent, and some evidence that certain adult learners still occasionally mess up minor grammatical details after decades of immersion. But other studies suggest that language-learning ability trails off gradually with age in much the same way that, say, physical ability does. A 35-year-old can still get in pretty good shape if they work at it.

I know plenty of people whose language-learning strategy is basically, "Put myself in situations where I need to use the language and I can puzzle things out. If I get curious, maybe look something up once in a while." It's not quite like childhood acquisition, but neither is it a completely different process.