Why do you think that it is so difficult for researchers to agree on the CPH (Critical Age Hypothesis). Give explicit examples from the readings. Also give examples from your own experience.
It is probably so difficult to reach a consensus because the research we have is not usually generalizable, and the results are contradictory at times. In addition, many researchers who may even believe that the CP exists try to prove it from very different perspectives or approaches. For example, researchers such as Lenneberg (1967)*, Selinger (1978), and Diller (1981) focus on the neurological development, or what happens in the brain as we grow older, from the point of view of how lateralization takes place, localization into the dominant hemisphere, and cell development within the brain, respectively.
Other researchers look at this intuively felt and believed-in phenomena from the perspective of loss of of or changes in one's acces to Universal Grammar as we age (Martohardjono and Flynn, 1995; Cook and Newson, 1996; and Hawkins, 2001), although even among such Nativists there is considerable disagreement over how much one can access at a certain age and how much of these abilities are retained. So, even though they are all proponents of one overriding school of thought, they are nowhere near a unified idea of what may influence the or confine the CPH.
Still a third group, including Krashen (1973 and 1975), Stengel (1939), and Guiora (1972 and 1992) relate the ability of languages to social and affective factors such as identity, attitudes and motivation toward learning the target language, and acculturation.
Because there are so many learning styles and strategies, so many developmental and affective factors, and such a wide range of reasons to learn a second language, individual results in gaining language proficiency in an L2 vary, just as the individual learners vary. There seems to be an age at which learning a language begins to become more difficult for the L2 learner; however, this is not a clearly defined, nor an absolute, truth. Nor has it been proven through research. Many post-adolescent language learners can become proficient in an L2, for many different reasons and within many different environments. The only (near) fact that comes out of CPH research is that younger learners almost always make much greater strides in adopting a native-like or near-native pronunciation, while many fewer adult learners do.
In my own case, I have seen a lot of anecdotal evidence that something like a CP exists, but I have also seen language learners who began learning English as an adult become highly proficient in that language. In the (paraphrased) words of countless other researchers and teachers in our field, I will beg off of making a conclusion and will say that further research is needed, and, most importantly, different people learn differently and there is no one right answer regarding the parameters or causes of a CP.
* All research cited here are citations directly from Singleton (2005)