I cannot disagree with that; what I would suggest is that if the adults spent as much time as the children immersed in the language and the necessity of speaking it, I suspect they would do as well if not better.
A few hours a week fumbling with a basic grammar guide is certainly better than a few hours a week immersed and desperate to communicate, but fifty hours a week immersed and desperate to communicate is what the children get and that will always do better than a few conscious hours a week with a basic grammar.
When I say "immersed", I don't just mean "surrounded". I mean it's coming at you, deliberately, and it's your only option for communication that you are desperate to engage in, mind spinning and whirring, latching onto constructs and experimenting with them, every human social fibre in your being that demands you communicate and connect with the people around you (and that is a very strong part of being human) driving the desperate urge to learn the fucking language, on the order of 10 to 16 hours a day.
I get that. I just don’t see any reason to suppose that the children are almost always more immersed than the adults in this sense. There are plenty of scenarios where the adults will have a more urgent need of that sort than the children. For example, my husband moved with his family to the Czech Republic as a teenager where he want to a French language school. His father worked as a diplomat. My husband still learned more Czech than either of his parents.
Well, what can I say? Perhaps your husband is some kind of language learning genius. Perhaps your husand's diplomat father was not so skilled at learning the language of the very nation he was working in. Your husband spent his workdays in school speaking French, his father spent his work days in an embassy speaking probably at least two and maybe more languages. Neither of them immersed.
I suspect, however, that for a typical immigrant, the child does not spend their days in a school taught in a non-local language, but instead simply goes to the local school, and I suspect that for a typical immigrant, the adult does not work in a building whose working language is their own native tongue. Your (husband's) experiences are somewhat offbeat.
I'm not making a claim about the typical experience, I'm saying that it's a near-universal that the children acquire the language faster. To me, that suggests that immersion is unlikely to be the main factor. Even if children are typically more immersed, I doubt this is the case anywhere near as often as it's the case that the children do it faster.
Your position is in accord with the prevailing scientific view on language acquisition. It's unfortunate that HN seems to be less than receptive, considering HN is supposed to be about intellectual curiosity.
A few hours a week fumbling with a basic grammar guide is certainly better than a few hours a week immersed and desperate to communicate, but fifty hours a week immersed and desperate to communicate is what the children get and that will always do better than a few conscious hours a week with a basic grammar.
When I say "immersed", I don't just mean "surrounded". I mean it's coming at you, deliberately, and it's your only option for communication that you are desperate to engage in, mind spinning and whirring, latching onto constructs and experimenting with them, every human social fibre in your being that demands you communicate and connect with the people around you (and that is a very strong part of being human) driving the desperate urge to learn the fucking language, on the order of 10 to 16 hours a day.