From what I keep hearing from other people transitioning between language contexts, that second sentence is what seems to make this the most asymmetrical—put an

From what I keep hearing from other people transitioning between language contexts, that second sentence is what seems to make this the most asymmetrical—put another way, the environment refuses to immerse you as an adult in the same way a child would learn. You won't get the same inputs, you won't get the same idioms; you might get more of a sanitized or limited version. This is something that I think happens not just with language, but with other traits—personality, skills, etc. A lot of these things can be more mutable than “expected” purely based on what a “self” can withstand, but the expectation itself causes friction and inertia from both directions: the person has to adjust their self-expectations, but also the social environment has to add energy and information to make those changes happen in a way that's integrated with the world. For anything that isn't adequately compartmentalized, sometimes there is no effective framework available; if the people around you already think they know you, either by broad categorization or by previous experience, you have an uphill battle. Children are more socially recognized as mutable, so both adults and child peers are more likely to put in the energy, and they're also placed in roles where experimentation is safe.