As a late starter, I have no doubt that my best performance came well after age 30, but I do wonder if I have passed my peak. I don't run enough regular events to see any trends, and don't do enough regular training to see any improvement. I am not convinced that stretching is that helpful, though I wouldn't argue with professional trainers. One exhaustive resource describes a balance between risk of injury from lack of flexibility and reduced performance from excessive flexibility.We go slower as we age, but we also age when we start to go slower.
Meanwhile, a couple of times a week, I would fire up an app on my phone and actually do some non-running exercise and stretching.
My opinion on stretching is it helps reduce injuries as muscles become tight from repeated heavy use. That's probably the same as your comment about balance, that stretching a tight muscle simply moves back to the middle. I think I'm still moving toward my peak, with improvements coming faster than my body is slowing down. Would I have been better if I'd started ten years earlier? Probably, but I was never going to be elite. I question if I have the physiology to qualify for Boston (I don't question if I have the determination or commitment to try to find out).