For help, we called Bonnielin Swenor, an eye data specialist and director of the Johns Hopkins Center for Research on Disability Health. Swenor introduced us to her friend and colleague Megan Collins, a pediatric ophthalmologist and researcher at the Wilmer Eye Institute at Johns Hopkins University, who seems to know all about eyeballs.

Collins points out a simple survey question we're using -- "Do you wear glasses or contact lenses?" -- covers a range of underlying conditions, from astigmatism to eye strain to the severe inflammation I want to make acceptable. But for the sake of simplicity (perhaps too simple), we'll focus on two common conditions: myopia and presbyopia.

Nearsightedness can affect young people: People who have worn glasses since childhood or adolescence often have myopia, also known as myopia, corrected. To people with myopia, nearby objects appear clear, but distant objects blur.

Presbyopia makes people old: It strikes almost all of us in our forties or later, blurring nearby objects while distant ones remain clear. (It comes from the Greek press, which roughly means elder. Yes, it shares its roots with the Presbyterian Church, which is a church led by elders.)

What's behind the curse of presbyopia? As Collins explains, the lenses of our eyes become harder and less flexible as we age, a trend some of us can perfectly understand. Because the eye muscles can't easily bend the hardened lens to focus, our eyeball's zoom function stops. Soon we were squinting at screens, holding restaurant menus half a mile (0.8 km) from our faces. That's when we put on reading glasses, a silly magnifying glass headset, or - if we already have glasses for nearsightedness or other ailments - bifocals.

According to the data, people start using reading glasses around age 40 and don't level off until around age 60. If we wanted a variable that best defined middle age, we could do a lot worse than the onset of presbyopia. (Here's an idea for a killer column. What else defines middle age? Let us know!)