I think a lot of fluency is having someone carry you around 24/7 and talk to you like a baby for a few years. Much of the rest is just having patient people aro

I think a lot of fluency is having someone carry you around 24/7 and talk to you like a baby for a few years. Much of the rest is just having patient people around you for another 3-4 years who will listen to you babble like an idiot and correct you when you're wrong. I agree that if I could do nothing but spend 3 years in total immersion, I'd probably accumulate an adult-level vocabulary (again, the ramp-up is steep), but I still wouldn't sound like a native at that point, whereas a toddler would sound like a native...but have a toddler vocabulary.