Make friends with Dave and Carson. Read and heed what they post.

I feel the need to weigh in here with some pessimism because I suspect you people are being a bit too rosy.
Keep in mind I am speaking from my own experience, which is that I started running in my early twenties and that, until recently, I played soccer at least once a week since the age of about 30 years.
I think I became appreciably slower at age 40 years. It wasn't really that I lost the capability of speed or of improvement. It was injuries. You start getting injured a bit more. It takes you longer to recover. All that interferes with training.
Until recently (last seven-eight years or so), I always felt like I could be as fast as I ever was on a good day. But, those good days just don't come very much. And, when they do, you have to make up for the loss of conditioning you have from not being at 100 percent before.
My brother and I facetiously refer to it as the "migrating injury syndrome." You just always have something wrong, maybe something niggling, maybe something serious, but always something. It moves around to different body parts. But, an injury is always there.
I am not going to say it is impossible to improve after age 40 years. But it becomes increasingly difficult.
My wife has had a hard time improving post-40 years too, and she has not had half the injuries I have. She is now 51 years. For at least the past 10, she has been working hard to improve her marathon time. Her last marathon? She finished with almost exactly the same time as the first marathon she ever ran, at about age 34-35 years. Despite serious work. Despite greater experience.