Octopus lives are short, and this genus is not the exception that beats the rule. Figures in print range from roughly one to two years up toward two or three. The spread is not sloppiness alone. Nutrition, water temperature, and the intensity of light in the place an animal lives all move growth and the calendar of reproduction. A number given to a tenth of a year is more precise than the animal.
What is consistent is the shape of the life, not the exact length. These are not octopuses that breed annually for a decade. They grow fast, they mature, they reproduce once in a serious way, and the generation turns over. Offspring of a brooding female are ready to mate on a scale of months to about a year, depending on which account and which species you are reading. The adult you see in a pool is not an elder of the reef. It is an animal near the only reproductive event it will have.
That speed is why a local population can seem to appear and fade. A good season of settlement, a bad season of storms or cold, and the number of animals in the shallows changes without any mystery beyond ordinary short lives. It is also why removing animals for the aquarium trade, or killing them out of fear, falls on a population that does not have a long adult reserve standing behind it.
If you are keeping a notebook, write the date and the place and resist the urge to age the animal by eye. A small one may be young, or it may be a small species, or a male beside a slightly larger female. The life is short enough that the distinction matters less than the habit of leaving the animal there to finish it.