Dear Wilmar! It is difficult to give advice. In such a situation should be carefully considered procedure and experience of experts in this field. Please see articles in the appendix to my letter.

All the best.

Vladimir

Try to correlate that with EEG measurements based on your own hypothesis - I have been using Emotiv Insight EEG headset and Python based control script applications, in conjunction with EEGLAB analysis on Matlab for assistance, in the field of stroke rehabilitation and it has been quite helpful. 

http://sccn.ucsd.edu/eeglab/

http://emotiv.com/

Dear Wilmar! Please do not get carried away only the analysis of electrophysiological data in the assessment of cognitive abilities. It is necessary to combine these methods with other tests, for example, psychological.

I think that if you want to assess an individual's cognitive development you should first assess his/her cognitive development mainly through developmental tasks, Piagetian tasks, for example. After this it may be interesting to see what happens in one's brain when, for example, one thinks in concrete operational or formal operational terms. As it is, it seems that your question posits, like most, if not all neuroscientists, do, that it is one's brain/mind that is, for example, concrete operational, or formal operational in Piagetian terms. I think that it is misleading, and even nonsensical to say, for instance, that our brain is cognitively, socially, or morally developed. It is a person as a whole, not his/her brain, who thinks in a preoperational, operational, or formal way in Piagetian terms, or in a pre-conventional, conventional, or post-conventional level in Kohlberg's terms, just to cite two well known developmentalists. Of course, it is more than natural to believe that different things (e.g., electrical firings) happen in our brain when we think, for example, in a concrete or in a   formal way (see above). Our brains, however, are only a necessary, but not a necessary and sufficient condition for we to think, for example, in an operational or a formal way. In short, to understand such a way of thinking we have to look not only at a certain space (e.g., our brain), but also (and mainly)  at a certain period of time, or, as  Wittgenstein put it at  one's language-games, contexts,  and forms of life. More to point, the  concept of development itself was not  coined by a certain brain, but by several theorists ( with non-impaired brains, of course).