Your eye is like a camera. To achieve focus on any camera, you have 3 options. Option 1 is what a healthy eye normally does, which is to focus your lens to a distance of an object you wish to capture.

But as you age, the lens in your eye loses elasticity and it's harder to change the distance to which it is focused. In fact, this function is the first thing to go for most people - the lens' ability to stretch and compress into a desired shape is progressively lost, which is why reading becomes harder.

This is like switching to a camera with a fixed lens, and is a main reason why surgery can't fix the loss of ability to see up close.

But if the lens becomes fixed to a narrower set of depths, you can still use options 2 and 3.

Option 2 is to increase the distance between the camera and the object to the one that your lens is already focused at. You've seen people do this (or may have done it yourself) when you moved the thing you're reading further away.

The last thing you can do is to increase the range of depths that your fixed lens is focused on. This is done by making a tighter aperture, which reduces light scatter*, thereby increasing the depth of field.

Your eye can control this in two ways. You can squint to tighten your aperture, or you can force your pupils to reduce in size by shining more light onto an object.

As you age, you can expect to be insinctively doing an increasing combination of all three modes of fixed-lens focusing. This is why those of us over the age of 40 buy reading glasses by the dozen - this is akin to carrying an extra fixed camera lens set to a closer focal distance.

*Not the most correct explanation for why the DOF is increased. It actually has more to do with ray angles, but it's close enough for the sake of this explanation.