I am 43. I started late and actually ran some of my best times upon turning 40. I did not really notice much the first two years of master's running. But I have noticed things the past two years.
For me the biggest problem is that I have always been getting faster thanks to my late start. So, putting in the workouts and all the effort was always rewarded. Now, I have leveled off and am not getting rewarded with improved times. I am still motivated to try to knock a few seconds of some PRs, but the idea of age group racing and minimizing loss of speed over time just doesn't interest me at all.
Physically, hard track sessions (200, 400, 800 5k pace and faster) tend to be pretty devastating. The next day I can barely hit 9 min miles. And I can run a marathon @ 2:50.
It takes longer to get warmed up, whether just on easy runs or in the midst of a workout. My first repeat of an interval session always makes me feel like I am not going to make it. If I am not doing a workout with a group, there is a good chance I will bail half the time because it just seems to be too hard an effort.
But you do not really slow that much in your early 40s. 45-50 is the big decline generally. I watched a local elite master go from 15:30 5k in his early 40s to barely breaking 18 in his early 50s. Quite humbling. Some guys resist the decline better than others. But 50 is pretty much the end of the road for feeling anything like you did when you were at your best.