My information comes from a China National team coach and also from a coach involved in the China Youth Badminton Development programme. If you have information from Zhao Jianhua (whom I see as one of the all time best players), regarding the ideal time to start training, I would seriously consider it. I believe you will make the best decision regarding your kid's future. As for the 11 year old girl doing well in a U-15 tournament, can't comment that much. Different places, different requirements. Players mature at different times and has to be viewed individually. Take Chinese players for example, Lin Ying, former China Ladies doubles and mixed-doubles player was playing for the national team at the age of 14, Han Aiping was representing China at 16, Wang Lin at 14, was competing in the Youth World Championships. And not to mention the Korean kids that came to Canada, a few years back for Canada Open and beat our national ladies team players.