The speed of their metabolism is related to water temperature: by keeping the water that cold, the Long Beach Aquarium extended the life of the bimac in question. This does not in any way mean that 59 degrees is an ideal temperature--far from it, you could argue that the Aquarium unnaturally extended the animal's life.
If we want to think about the "ideal" temperature, the starting point of our search should be the temperature range of the animal's natural habitat. Lets see if Google can help.
As you can see from this, the water temperatures off southern California seem to vary from a winter low in the upper 50s to summer highs in the upper 60s. With the internet, the world is at our fingertips!
edit: I think the point here is that under no realistic circumstance should you expect a pet octopus to live for two years. Very generally speaking, these animals have a simple life history: live for a year, lay eggs, die. Most successful bimacs have been sold by a breeder at 4-6 months old, meaning you can only reasonably expect 6-8 months with one. From a practical standpoint it really is less, assuming multiple weeks of senescence and the fact that most hobbyists can't match the cool water temps in the summer. I try to chime in whenever I hear someone bandying "a year and a half" or "two years" around here or on Reef Central, because this is just plain unrealistic. It is unfortunate but true and too many people pursue this hobby with unrealistic expectations. People need to be honest with themselves and decide if it's a deal breaker that their pet will only live a few months.
I think if you are talking about bimacs, although they can survive at room temperature( above 70 deg F), they will thrive at their native and natural temperature range of 54-68. Clearly there is a lifespan range, and the experts feel their average lifespan is one year. In my limited experience, I have not been able to identify the factors leading to growth and size variations and time to senesence and death when I controlled for variables like food availability and temperatures. I would be interested in tracking all of my 100 octopuses, 70 of which have been sent around the country. I know some have already died-they would be 10.5 months old now. Personally I keep my aquariums cool with chillers. Currently, one tank is at 57F with 6 in it; 3 others are set at 65F with about 10, 5, and 5 bimacs; the smallest tank is 125 gallons. Colder gives you an advantage not only in relative life expectancy because of the metabolism but also gives you a little wiggle room for human errors as in water quality issues. We will keep everybody abreast of life span as the captive raised bimacs reach the end of the lives. No eggs yet but I am hopeful and I feel they (the females) have mated- if only it was easy to tell them apart. For those of you coming late to this website I would refer you to the June 2006 TFH article I wrote about captive raising of my bimacs. Zyan