This may be true. I believe that the existing language knowledge of an adult is sometimes a handicap, though, at least in early stages. It is hard for an adult to “empty his cup”, and truly think in another language. For quite some time, adults tend to translate mentally to their first language, and this can be a huge obstacle to true fluency. At least, this has been my experience.
Also, an adult would, as you say, need to apply himself strictly to a carefully comstructed, unnatural program (flashcards, grammar learning method, etc.) in order to learn faster than a child. If the child learns by nothing more than immersion, then the child has the easier time of it (although perhaps not the faster). To me this means that children are naturally better language learners.