1. That will depend on how you use them. ie, how hard you discharge them or how low you discharge them. Less amp draw and not letting them get below 3.4v per cell will lengthen their lifespan.

2. Old packs will take less mAh to full charge. Internal resistance will get higher over time.

3. Yes, you could loose power due to old batteries that are getting bad.

4. I don't know of any kind of testing except for checking the internal resistance that some chargers have.