If A learns to play the piano starting at age 5 and B starts at age 35, I wouldn't be surprised if A is not only on average, but almost always, better at age 25 than B is at 55. Unfortunately, that's basically impossible to study while controlling for all confounders like general intelligence, quality of instruction, and number of hours spent on practice.

If all you are saying is that people who start learning a language at age 2 are almost always better at it than people who start learning the same language at age 20, I don't think anyone would disagree. The whole discussion is about controlling for confounders...