Comment by raverbashing

Being surrounded by talkative people helps a lot, and then you get to learn the actual constructs, not the theoretical grammar. Not being facetious, I think that's the main thing behind "it's harder to lear a new language as an adult". I think it's less due to ability and more to do with the contexts and opportunities to practice.

(Though yes, by how they learn you notice what they get easily and what confuses them)