Let's look at today's Dell page. I see 300 cycles or 18 months. You are well over that so I'm not confused at all here. You should get a new battery. No, it ages just as fast on the shelf or in use. About the only thing that slows the aging is to charge it up then place it in a ziplock bag (moisture control) then in the refrigerator for those years. Sometimes helps but it's a chemical thing. Think about it.

As to mAH, the more the better if you want more minutes but the same degradation happens over time and cycles. It seems to annoy a lot of people that these only last a few years in or out of use.