I took three years of Spanish and got an A every semester. Even when it was still fresh in my mind, I was nowhere near able to hold even a very simple conversation. And now just a few years later it's all totally gone from my brain.
My mother's native language is Spanish and she never taught me, which I resent her for. But I still find it incredible how shitty my public school education in Spanish was. We really should be teaching kids a second language from kindergarten up.
Language acquisition research on a "critical period" for language learning is inconclusive. Neuroplasticity may make it easier for a child to acquire/differentiate specific linguistic information (e.g. sounds that exist in one but not both of the languages) but being socialized into a second language discourse community /also/ means that they're getting far more time practicing the language.