Keep in mind the brain's disposition to learn changes depending on age. And as an adult your brain will struggle for many different reasons: stressful day, lots of other information streams, far less time for learning, decreased flexibility, etc.

As a kid I learned 3 languages (including the native one) with relative ease. As an adult I really struggled to learn the fourth using what I think are the same techniques. Given the factors above the reality may be that it's almost impossible to learn in the same conditions.

Yes, but they had to teach me how to read later in life. This is a two-for-one strategy, because the reading skill transfers much more easily.