It depends on how the batteries are maintained since [Lithium-ion polymer batteries need to be used and recharged regularly](https://www.apple.com/batteries/). There will definitely be some reduction in capacity as time passes, regardless of whether the batteries are regularly used/charged or not.

From [Battery University - Is Lithium-ion the Ideal Battery?](http://batteryuniversity.com/learn/article/is_lithium_ion_the_ideal_battery) (emphasis mine):  
> Aging is a concern with most lithium-ion batteries and many manufacturers remain silent about this issue. *Some capacity deterioration is noticeable after one year, whether the battery is in use or not.* The battery frequently fails after two or three years. It should be noted that other chemistries also have age-related degenerative effects. This is especially true for nickel-metal-hydride if exposed to high ambient temperatures. At the same time, lithium-ion packs are known to have served for five years in some applications. 
>
> *Manufacturers are constantly improving lithium-ion. New and enhanced chemical combinations are introduced every six months or so. With such rapid progress, it is difficult to assess how well the revised battery will age.*


References:  
[Apple - Lithium-ion Batteries](https://www.apple.com/batteries/)  
[Battery University](http://batteryuniversity.com/)