Cellular regeneration degrades as an organism ages because DNA strands have sequences at their ends called telomeres. These are relatively arbitrary sequences that don't really serve any other known purpose. When a strand of DNA is replicated, a section of the telomere is lost during each cycle of replication at the 5' end of the lagging strand's daughter.
 The telomeres degrade over time until the Hayflick Limit is reached, which is essentially the limit of times the DNA strand can replicated. Now, there are some cells (cancerous cells are an example) that have active telomerase enzymes that regenerate the telomerase, so their regeneration limit is just based on the resources available. Without active telomerase, the DNA loses imperative parts of the sequence and can't be translated properly. 
 For your other question, it's a little bit more complicated! The terms warm blooded and cold blooded aren't very accurate. Rather, there are endotherms, ectotherms, and heterotherms. These organisms regulate body temperature from inside, via the environment, or a combination of both, respectively. Ectotherms typically have a lower metabolic rate because they maintain homeostasis at different temperatures based on the environment. They require less ATP for this to occur, so metabolism is slower and they have less mitochondria in cells. This allows for slower replication, which means longer lifespan. 
 Hope this helps, correct me if I'm wrong about anything