Octopus age is very hard to determine in live animals and takes special equipment for post mortem determination. There is no reliable average size even among siblings. Sexual maturity can be determined in males by the way they carry their third right arm (clockwise as you orient your eyes with the animal). In O. briareus, that would be at about 5 months (probably about the same for O. mercatoris from memory but mercs may mature a bit earlier). Abdopus live (in general) a bit shorter lifespans so I would guess 4 months to be close. Animals in senescence often lose the ability to color strongly but without prior comparisons it is not easy to detect.
That being said, most of the animals coming from the Philippines have been sexually mature, many have been full adults but we have seen a few juveniles.
An arm length of 7-8 inches is quite short for an adult aculeatus. There are others within the complex that this arm to mantle ratio that are more likely. Take a look at @davelin315 's new animal and my notes in that thread to see if your little guy looks similar.