I have been reading “The Mind and the Brain, Neuroplasticity and the Power of Mental Force” by Jeffrey Schwartz and Sharon Begley. What becomes clear is that the brain constantly responds to stimulus to adjust and create neural networks. Language learning is a form of training of the brain which can take place at any age.
It is clear from research that adults retain brain plasticity and can continue to learn languages all their lives. It is more their anxieties rather than any inability that holds them back. However, they should not be treated like children in the classroom. Ideally the adult is more self-directed. Certainly the self-directed adult learner is the target of The Linguist system