My wife and I currently have the pleasurable duty of looking after our 11 month old grandson once or twice a week. We basically talk to him all the time when he's not asleep, and I'm sure his parents do the same.
I was "listening to myself" talking to him earlier today and realised that even in a fairly short time, it involved a lot of words. So over the pre-school period in total, that must amount to a massive number of words a child can be exposed to. I'd always kind of known this in theory, but it's a long time since our kids were at that stage, and it's only just this recent experience that has really brought it home to me.
So I think adult learners definitely need to try to maximise their exposure as well; but I guess we all knew that already.