This was commented by Steen of Frost, I con't remind which one, during a match. You need a certain thousands hours of training to reach your peak level - something like 6000 hours - studies said, and CHN players reach that level at around 22 years, while DEN players reach this at about 26 years. They start serious training more late, and training method is different. Thus normally CHN players would tired out younger. Another aspect is that CHN players live like young students in dorms at the training center. This kind of life suit some but is a bore for others. At a certain age, when you wish to marry and have a more private life, it is quite an incentive to call it off. Moreover, I would think that today's badminton involve more tourneys, more travelling, more every thing.