First off, I have not read the manga, and I will be making sweeping statements that may not apply to the manga.
That said - Lets go through the evidence.
#Technology#
Technology
- Human-like AI is both possible, and demonstrated in every incarnation of GiTS, though even the most sophisticated examples like proto tend to lack cyber-brains.
- 'Ghost-dubbing' or copying a cyber-brain is also possible (and demonstrated), but the copy is inferior.
- A cyber-brain would also be pretty heavily augmented to be able to interface seamlessly with both the cyber-bodies and external machines. You also see them outside of the body from time to time in their metal cases.
- The arise series also introduces a couple of characters that have used 'AI chips' to replace brain functions in their ailing brains.
#The major#
The major
###Origin story###
Origin story
The major's origin story varies from continuity to continuity (arise/stand alone complex/the manga and the movies all exist separate from one another, in spite of their similarities)
- In arise, the major has been a cyborg from birth (or at least a kind of brain in a jar).
- In Stand alone complex a plane crash left her in a coma; forcing(?) the use of a cyber-body to revive her.
- The movies do not make any specific mention of her origins to my knowledge.
###Age###
Age
- Old enough to participate in the last big war... The wiki article on the major suggests 31 but i'm not buying it.
###Body###
Body
- Military-issue cyber-body with cyber-brain.
###Capabilities###
Capabilities
- One of the best hackers in the world, and having control over her cyberbody above and beyond most.
#Conclusion#
Conclusion
The major probably isn't an AI judging solely from the fact that she predates the indistinguishably-human variants AND has a cyber-brain.
It is however quite possible that the major isn't the first... heading section nine is rather dangerous after all. The technology does exist to make copies as well as offset the ailing abilities of poor copies with AI chips, perhaps even pushing her abilities past her otherwise (relatively) normal contemplates.
In all, I would argue that the major is (technically) human, though your mileage may vary in a what measure is a non-human kind of way.