Sure there are still some top players who have expanded waistlines and avoid the gym like the plague, but, especially since Tiger came on the scene, most top golfers work very hard at staying fit. The modern swing and the modern power game demands a high level of fitness as well.
I note that Jason Day is speaking about the possibility of retiring at the age of 40. I think this should come as no surprise to anyone. As hard as he works, as much stress as he puts on his body with his swing, he will be fortunate to still be competing and at the top of his game at forty. In fact, I'd be very surprised if he was.
Harvey Penick said that he preferred watching the swings of the guys on the Champions tour--the swings that had stood the test of time. He suggested that many of the modern players, with their modern swings, wouldn't be around to play on the Champions tour. There is a price to pay for golfers who adopt the modern swing that relies so much on rotation and core strength. They will wear out much faster than the guys who still swing the club, like Vijay Singh, Colin Montgomerie, Phil Mickelson and Angel Cabrera to name just a few of my favourite swingers.
So, will Jason Day be done at forty? You can almost bet on it. Unless he changes his swing. I should say that both my old mother and I are big fans of Jason Day. He's a tremendous player and a fine person. I am not trying to be critical in any way of his swing either. Obviously his swing is one of the best swings in the game. It might be the very best swing in the game. But it is a young man's swing.