effect of age on battery performance

My understanding is its not as big a deal as you might think. The DOE drove a pair of Gen I Priuses (along with other hybrids) 160k miles and then evaluated the condition of the battery. As I recall they found the batteries down almost 50% on capacity, but there was still almost no effect on the mileage performance of the vehicles. Its also my understanding that the Gen II batteries are of a more refined design and should last even longer. For the same reason, just adding more battery capacity has almost no affect on mileage under most driving conditions. PHEVs don't see any significant improvement unless they are charged up from the grid.

As far as straight life-span (independant of # of cycles/miles) I believe NimH are generally very good. Much better than Li-ion, which I understand to be one of the challenges of adapting Li-ion for hev/phev/bev use. Actual numbers will vary greatly according to specific design optimization, but the Prius batteries are generally designed for very long life. The Japanese product brief for the battery modules doesn't list a specific number, just says "[SIZE=-1]Much improvements on reliabilities for long life due to cell stack construction."
Plastic Case Prismatic Module | Panasonic EV Energy Co., Ltd.

Rob

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Something to keep in mind about battery life is that the Prius doesn't use the complete capacity of the battery like most of the devices that you are used to. The SOC (state of charge) is kept between 40% and 80% by the HV ECU programming. The battery is really used as a buffer to capture energy that would otherwise be wasted.

This is totally different than battery use in a Power tool or a Laptop where the battery is run down to the point where it can no longer provide power, then recharged to 100%. Both of those acts shorten battery life.