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According to the original paper, 81% of SO users didn't provide their age, so to conclude that older programmers are more knowledgeable from this data seems unwarranted. The paper acknowledges however that SO population may not be a good proxy for the programming community in general:

Research into the nature of the SO population and its relation to the programmer population at large needs to be conducted, in order to support inferences to the programmer population. Further investigation is needed of how SO measures, such as user reputation and question scores translate in to programming knowledge and ability. Research on the relationship between age and knowledge on SO should be related to the existing literatures on aging and on the development of expertise.[1]

[1]: http://people.engr.ncsu.edu/ermurph3/papers/msr13.pdf