Why does focusing on nearer things cause myopia? See if you're curious at even a basic level, you'd realize that there are important *details* about stuff like this where it actually helps to have some actual subject matter expertise and knowledge.
Explain how, please.
A certain age -> which one? Why?
your eyes determine -> How? What molecular growth signaling pathways are involved? How do they integrate with your brain's visual processing centers and how does that relate to "how well [your eyes] focus". Is there a biomechanical signal from muscle stress or eye curvature?
How would you test this? You'd have to change this process somehow to show that the effect is real, but you obviously can't do that with humans, so you'd probably have to use mice, but their eyes are different, but how so?
Without any of this information, it's a nice "just-so" story about cavemen looking at the horizon, but not much more than that.