Per this article, adults can learn languages to a high level nearly as fast as children.
One popular belief about language learning is that if you start learning it as an adult, rather than as a child, it will be much harder to reach a high level. It turns out that it is harder on the average, but not much, and there are plenty of adult-starters who perform as well as child-starters.
I found this article (and the study it references) to be very interesting because it covers native-like fluency, rather than just the lower levels, and tests people with decades of experience. In fact, very few learners reach this level in less than 10 years.
Another thing that is mentioned is learners can reach a very useful level in a language in a year or two. Although it will take over 10 years to be native-like, you can get 80% of the way there in 20% of the time (80/20 rule).
I hope that the results of the study will encourage adults to stop thinking that they cannot learn a language, and start learning!