But by as early as the mid 20s, the eye has lost about half of the range of focus, which is measured in units called dioptres. Where a child of 10 might have 20 dioptres of accommodation, that range will be halved by the time he or she reaches the quarter century mark.
"At age 25, I'm already half-way shot, which is sort of interesting because you don't notice it," Kline says.
It does become apparent by the 40s and continues to decline, albeit more slowly, until about age 60 at which point "that's it," Kline says. "There's nothing left."
While it is hardening, the lens is also yellowing - affecting the aging eye's perception of some colours - and becoming more cloudy. The latter phenomenon is what we know of as the formation of cataracts. If we live long enough, most of us will undergo cataract surgery to have our cloudy, yellowed and hardened lenses removed and replaced with artificial intraocular lenses.
Currently, those artificial lenses are not flexible and therefore can't restore accommodation. But researchers are trying to develop a new generation of the devices that can bend, meaning someday cataract surgery might also reverse presbyopia.
"You'd solve two problems at once. One is you clarify the lens right away. And the other is you would restore ... the ability to focus again," Kline says.
In the meantime, people who hit the presbyopia stage of life have a number of options, says Vancouver ophthalmologist Dr. Simon Warner.
People with good distance vision - in other words, people who don't already wear glasses or contacts - will need reading glasses. And yes, the inexpensive ones at the local drugstore will probably do just fine.
"There's nothing harmful about those glasses," says Warner.
He suggests, though, that people first see an eye professional for a checkup to make sure everything else related to their vision is in proper working order. In the process, they should ask if they can simply use off-the-rack reading glasses and what strength they need, he advises.
"Some people need more strength in their reading glasses than others," explains Warner, who has a practice but also teaches at the University of British Columbia.
"And sometimes it depends on what people do. If you're a gardener, you might not need very strong reading glasses. But if you're sitting in front of a computer all day, you might find that you need reading glasses more frequently."
People who wear glasses for distance will need bifocals or even trifocals - glasses that allow the wearer to focus at a distance, at mid range and close up.
For those who go the bifocal route, Kline has a word of warning.
People who've been wearing glasses can have real trouble negotiating stairs when they first start wearing bifocals, he says. That's because when they look down to check the location of the stair, they are actually looking through the portion of their lens meant for reading. As a result, the stair could be a blur.
Contact lens wearers can also switch to bifocal contacts. But they may have another option open to them as well, Warner explains.
For some, under-correcting one eye - using a contact lens that is slightly less powerful than the eye actually needs for distance vision - will allow them to get by without reading glasses. The properly corrected eye brings distant items into focus while the under-corrected eye allows the wearer to focus at close range.
