Mid-40s here. I am more cautious about many things in life. I think the trigger was having a child. In answer to your specific questions, I think becoming more cautious is related to experience--seeing bad stuff happening to people we know. There may also be some hormonal changes--I'm thinking about risk taking behavior as a mating strategy, particularly for young males (note that I am totally making that up and have no scientific documentation to provide). What can we do about it? Well, you have to find balance. Don't sled on the same hills that you plunged down 20 years ago. Pick something tamer. Who cares? As long as you are having fun and staying safe, I don't think it matters if you slow down a bit.