I also have a comment related to the children vs adults learning language faster. I will begin by saying that I have a linguistics degree though I have not specifically read up on studies looking at speed (in years or hours, anyway) of learning, so I am curious also about this topic. If you have mentioned in a later podcast I will get to it, but can you possibly share some links to the studies you found? I am curious also whether those studies are looking more at classroom based learnjng, full immersion learning, or just conparing first language vs second language acquisition.
Related, however, there is some feedback I do have experience with. So, related more to your discussion about needing to learn a first language before a certain age to avoid other mental deficiencies, there is a critical period of language learning (different scientists will differ on what ages they say that is, though it is more of a sloping chart than an immediate drop off anyway, so it depends more how you classify it, I have seen age 5 being the cutoff and in school I was also taught age 12, so yeah, varied), and like I said, this is related to the brain needing a language thing, BUT it goes even further than that, and it ties into why it is important to teach languages to children rather than adults no matter the difference in time it may take to teach them.
Chiefly, any language learned during that critical period can be learned to native fluency, and can be learned much more easily than any language learned later, especially as an adult. This is why when a family with kids moves overseas, even if the whole family is fully immersed, the kids will learn the language better and much more easily than the adults, generally. This is a major reason why you will often see immigrants getting their kids or grandkids to communicate with people and stores and the doctor's office and such.
In addition to that, for every language learned during that critical period, it becomes easier to learn more languages after that period. So for example someone who grows up bilingual will find it much easier to become trilingual and beyond than someone who grew up monolingual. That said, kids who are learning more than one language growing up will often learn each individual language to full fluency a bit slower, and will often have gaps where they fill in with the other language until the learn both languages fully, but as long as they stick with all the languages they are learning they will not only learn all the languages in question as well as any other average native speaker but it will also help them in other areas, especially math and science.
Anyway, thought that was worth feedback, always love when you talk about linguistics, please do more.