My husband started playing when he was over 50, and we played almost every day the first summer and fall. He's found that he has more problems if he doesn't play regularly. Knee pain can be a problem, and if he's walking regularly, then that strengthens the muscles, and makes daily golf much easier. Over 40, I wouldn't just start playing two rounds a day. But if you work up to it, and don't have physical problems, it doesn't seem like there's a reason why you shouldn't be able to play two rounds a day.