There is a clear statistical advantage in starting to learn a language earlier, but that doesn't mean that late learners can't achieve fluency. In fact many can, and some may approach native speaker level.
It is pointed out that one reason why children improve more quickly is probably due to the fact that they are often exposed to the new language for longer periods. But up to the point of general fluency, adults are actually in many ways better at learning a language. And to appreciate the more subtle grammatical points with the accuracy of a natural speaker will take a decade or more, no matter how old you are when you start.
Quoting from the original report;
“What is remarkable about language is that we are (nearly) all extremely good at it, including adult learners.”