Going on the thin ice of hypotheticals, since cephs only reproduce once, and don't care for their offspring after hatching, it would seem that they have a lot less benefit from staying healthy after reproducing. Not that they have evolutionary pressure to die immediately, but they have a lot less pressure to survive than animals that might reproduce again, or that, like mammals, have to raise their offspring until they can take care of themselves. These sorts of discussions are always more complicated than they seem, though: before reproduction, there's a lot of survival and fitness needs, and there's not a lot of incentive to "switch off" all that, either, particularly since the young migrate away from the parents, so the parent isn't taking up resources the offspring need. There might be incentive for the female to get as fit as possible, then switch off and only take care of the eggs, but it's harder to see that in the males. On the other hand, maybe the males just have inherited the this aspect of the females' physiology, since they don't have reason not to, just like men having nipples...
Going a bit far afield of the original topic: we don't even know, AFAICT, what genetic or epigenetic or environmental control causes an octopus to be male or female; they don't have analogous X and Y sex chromosomes that anyone has discovered so far, and many of their gastropod relatives are hermaphroditic. Steve O pointed me at an article on a South African squid species where a few individuals show both male and female traits, so there's some opportunity in at least one ceph species for gender ambiguity.
Since it's been found that the octopus optic gland influences the onset of sexual maturity, I have to wonder if there is or was some evolutionary advantage to having the animals recognize favorable conditions for reproduction, and then flip the switch to go into full-on breeding mode. The mass spawning of some squids seems to be an example of this, but it's less clear why octopuses might do it; perhaps it's carryover from much older cephalopods? Of course, nautlius and cuttlefish don't seem to do anything like this, and lay eggs over a long period, so it's not all cephs. Does anyone know anything about Vampyroteuthis or Cirrate octo reproduction? Or Spirula?
It is essentially the same everywhere. We humans die off after we pass breeding age, too. 
