Chen Long, The Dragon, has just turned 28, born 18 Jan 1989, and he is still very much in his prime.
For such a physically demanding sport as badminton, physical fitness and stamina and endurance, is vital, one of the key elements to success at the top.
It's unrealistic to expect a player in his mid-thirties not to be anyway affected by waning physicality, and, despite the advancement of modern science at present, there is no way he is not at a certain disadvantage compared with a man in his mid-twenties, barring unforeseen circumstances such as injury and sickness.
Perhaps in the not too distant future, medical science is able to slow down the aging process considerably and extend the human lifespan by much, then, possibly, a mid-forties or mid-fifties athlete is equivalent to someone in his mid-twenties or -thirties today, greatly reducing the physical differences. It may happen sooner than we expected.