Because your eye's lens stiffens with age - presbyopia - and the mid-forties is its normal arrival time.
The lens focuses up close by changing shape; its elasticity decreases slowly over the years, so near focus fades. Most people need readers by about 45, and the power climbs until around 65 to 70, then levels off. Nothing you did causes it and nothing prevents it - not screens, not dim light, not wearing glasses early. Diabetes, cardiovascular disease, and multiple sclerosis can bring it younger. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Why do I need reading glasses at 40?
Because your eye's lens is getting older, not weaker from anything you did. Presbyopia: the lens focuses up close by changing shape, and its elasticity decreases slowly with age - so the ability to focus on nearby objects fades. It typically becomes noticeable in the mid-forties; most people need reading glasses by about 45, and the power keeps climbing until around 65 to 70, when it levels off. Nothing causes it and nothing prevents it: not screen time, not reading in dim light, not wearing or not wearing glasses. It happens to people who never needed glasses before, which is why it feels sudden. A few things bring it earlier: diabetes, cardiovascular disease, and multiple sclerosis are associated with younger onset. Reading glasses are the simple fix; an eye exam confirms it is ordinary presbyopia and not something else.