Critical Period Hypothesis Article by Wang and Kuhl

I am interested in the critical period hypothesis, so I was thinking this article would be interesting to read. It is titled Evaluating the"Critical Period" Hypothesis: Perceptual Learning of Mandarin Tones in American Adults and American Children 6, 10, and 14 Years of Age by Yue Wang and Patricia K. Kuhl. I felt it was well laid out by introducing you to the experiment, method, results, and conclusions. The article was testing the critical period hypothesis. They were testing to see if adults could recognize the difference in Mandarin tones. By seeing if they could make these recognitions it would show evidence for or against the critical period hypothesis.
There was a trainee group and a control group. Basically what happened was they gave the participants an introduction and pretest - they taught them anything to see where they were starting off. The testing consisted of sitting at a computer with head phones on to listen to the tones. Then, they gave them training on the Mandarin tones. This was followed by a post-test. The procedure for the children and adults was identical.
The results found significant improvement of identification of Mandarin tones. Also, the test found no abrupt decrease in the degree of improvement. The increase was comparable across all four age groups. The study did not support that there is a critical cut off period.
I thought this study/article was just OK. It didn't really show any new evidence or anything that hasn't already been said. A problem I found with it was the ages they selected for "adults". The age for the adults they tested were 19 years old. I guess they are technically adults and are past the cutoff age of the critical period hypothesis. However, when I think of adults in terms of learning a language I think of someone a little older, at least out of school. I would have more confidence in this study or would have found it more interesting had they used a wider range of adult ages like they did for children. (They use 6, 10, and 14 years for children and only 19 years for adults.) I personally do not think there is a cut off period with language learning. I believe anyone can learn a language regardless of age. I just think as you age you begin to think differently and take on different characteristics which may mean that you learn a language a different way than you would if you were a child. I also have to wonder how well a six year old would do on a test on the computer. Could they sit and pay attention long enough for these results not to be flawed?