It cannot be said that this ability deteriorates continually as you age. However, you reach a plateau. Adults can learn another language, but a completely different teaching style is required. You have to rely on the structural knowledge of the first language and transfer that to the second language. Children do not learn that way. Adults learn that way. What are the similarities? Well, if you are learning Spanish, and you already know French and have some knowledge of French grammar, you can learn by transferring your knowledge. The teaching method has to focus on that.
On April 2nd, 2009. See this statement in context.