I'm 52, soon to turn 53. I've been back at steady training now for about 5 years after a long hiatus. My sifu, Tony Brown, is 57. He just came back from Taiwan where he and his Norwegian team competed in the 10th World Cup Kuoshu Tournament. Sifu Brown came out of retirement there and fought in the full contact Lei Tai matches. Despite waking up sick that morning, he won a silver medal for his Norwegian team. I don't think they had him only fighting other 50 somethings...... Oh, and on top of that, he only has one arm. http://www.tromso.hungkuen.no/taiwan7.html
Granted, Sifu Brown has been teaching, training, and learning for a long time. He is very good at what he does. I would never consider going up against someone half my age in a full contact match, but he is just demonstrating that age is not necessarily an absolute, just like he has long demonstarted that having a "disability" is not an absolute. I have learned the hard way on how to adjust my training for my age... I herniated a disc almost 2 years ago training like I use to do in my early 20's. I don't look, or move, like I'm 20 years old, but I sure look a hell of a lot better than a lot of other 50 somethings who have difficulty picking something up off the floor.... and a lot of 40 somethings, come to think about it.
50 and above is not a barrier so I don't see that 40 something is anything to sweat. It is all in intent.