Age, combined with repeated cycles of charging and discharging, has an adverse effect on the components of the battery. Specifically, the active ingredients of a battery change and transform.
When a battery has been put through multiple cycles, as is expected in normal operation, the bond between the active material and the plate grids that are holding it in place weaken. Every time a battery is discharged a small portion of the active material falls to the bottom, reducing its overall capacity, this is known as shedding. This material slowly accumulates at the base and will eventually lead to battery failure. Either through the complete shedding of the material or through the accumulation of the active material at the bottom of the casing; shorting out the opposing charged plates within the battery.
The rate at which a battery will degenerate is dependent on a few factors: