I think this is a really interesting and counterintuitive suggestion that potentially makes a lot of sense. I'd never considered this. I always assumed the difference was just that babies' brains are different, but it's true....as an adult most language learning centers heavily on the written word, and that was not how we learned to speak our first languages. We learned to speak and listen, then to write. Adults do it mostly the other way around. Very interesting idea. I'll try this with my next language.

Anecdotally, I'll say I've been learning a second language for 20 years by myself in a place where that language isn't spoken by anyone, and I've made more progress in the past 2 years than the preceding 18. What I changed:

- 15 minutes on anki vocab flashcards every night

- reading books in that language daily instead of english (my native language) and making flashcards for every word or phrase I didn't know

- listening to videos and podcasts in that language aimed at native speakers, NOT learners, and watching them REPEATEDLY until I could understand as close as I could get to 100% of it without subtitles. I try to spend 20-40 minutes a day listening. I especially put on podcasts in the car, but I have to be at home on my pc to rewatch with subtitles or a transcript to get the parts I couldn't understand initially.

It takes a lot of patience. There's no way around that.