I would like to share my thoughts on the WC scholarship exams and hopefully put a few things in the right perspective.
Lengthy dedicated preparation is probably not going to help that much since the Election papers are not really testing for knowledge but they tease out hints of thinking capacity, depth of curiosity and perhaps degree of geekiness. The child needs to be able see the world around him differently and question "accepted wisdom" (even if that still prevails and even if it means rough-rocking the boat). To me, that may well be the biggest difference between EC and WC and the kind of boys they look for. Just 2-3 years of preparation will not work if that was for the sole purpose of getting "elected", but 2-3 years of learning for them is way too long. In fact, I suggest 9-month absolute maximum to avoid burn-out.
It is probably accurate to say that the knowledge required to ace the exams would have been learnt by the age of 9-10. I would say a 10 year-old acing an age-appropriate exam is probably a good starting point, but I think the expectation of having an 11 year-old acing the common entrance is neither necessary nor pertinent. Given enough feeding and enough practice questions, many can and will do just that, but it will not help with a question seeking a well-argued opinion about Japan's preparation for a tsunami and the handling of its aftermath, nor finding the demonstration of number theory manifested inside two simple additions. Acing the Common Entrance at 11 will almost certainly help towards acing the Common Entrance Scholarship exam though.
The Election has four compulsory papers and three or more option papers. The compulsory papers are General 1 (no parent has seen one but said to be like an IQ and general "awareness" test), English, Science, Maths 1, and the electives (min of 3 "but 4 is reasonable") are General 2, Maths 2, History, Geography, French, Latin and Ancient Greek.
Many of these papers have questions that do not have right or wrong answers or often the questions are incomplete so that candidates have to come up with a well thought out and well debated opinion and justifications for the assumptions used for the missing pieces to complete the question. When I saw them for the first time, my knee-jerk impression was a kid's version of the old Cambridge Entrance papers. There are lots of grey areas and topical current affairs derived questions.
For maths as an example, many of the questions require mathematical knowledge and skills a 9-10 year old would have already learnt, but they are typically put in such a way that requires not just a clear head and sharp mind, but a genuine understanding of the concepts and reasons behind the maths. Again, from direct experience, the vast majority of schools and even well respected tutors focus on teaching methods and knowledge rather than understanding - these questions are suddenly perceived to be ultra-difficult. It is shocking that some maths teachers and tutors could not solve some of these problems. Also, many questions have several very different ways of getting to the answers.
Science has a practical section. The experiments are perhaps to do with quite advanced topics and theories, but they give you step by step instructions and remove all the complexity a 12-13 year old would not have come across. Then they ask you to speculate explanations and theorise the results/findings.
Languages are again about understanding them enough to grasp subtle meanings and unusual vocabulary but there is clearly no expectation of getting the child to study dictionaries. However, a wide exposure to a diversity of topics, books, genre would definitely help - I suspect they are looking for hints/evidence of curiosity here too.
As for Gen 2, History and Geography, interpretation of data and ability to speculate anything with a solid basis is important. The opinions required are typically not what you could read in a book or news article and reuse.
The frustrating part is that most of us grew up being tested for knowledge and there is really not much anyone can teach a kid for these papers. Doing a couple of past papers will definitely help immunise the child against the shock factor and get used to the style. The topics, questions and problems are just unpredictable and left-field (which can be fun to those with the right attitude and mindset I imagine).
Most importantly, it is about ensuring that he really understands the most basic concepts so well that he could explain them to say his younger siblings. Then let him chill, spent time on his hobbies, discovers things to satisfy his curiosity and spend time on the most ridiculous questions he has to ask about the most trivial and childish subjects. None of the papers will benefit from studying or regular tutoring. It would be too cruel to make an 11 year-old work for a 90+ score in the 13+ exams as a prerequisite for WC Election.