The problem with the lithium chemistry is, age will affect them, used or not. they grow dendrites over time, and if the battery is taken care of properly, they'll grow slowly.
the thing is, industry wide, it's generally accepted that about the 8 to 9 year mark when these batteries will crap out. They'll grow long enough to eventually short out the cell, then the pack is dead. It won't be a degrade really, it'll pretty much, the pack now is crap, it's done and itll happen very fast.
The longest warranty you will find typically is 8 years, that's because they know it won't go much longer than that, on average.
Here's the thing, is your battery a day from death or a year? no way to tell honestly. It could last another two maybe 3 years, it could die tomorrow.
In other words, with proper care, you'll get 8 years out of them, any time after that, is a bonus.
For what it's worth, the dendrite growth tends to happen during charge / discharge cycles, so in theory, lower cycled batteries, should be in a bit better shape.
Aaron
