But....but....it's still just cloned human flesh no?
no, because
cloned would mean full working muscles and widespread blood system, while T-800 just has a layer of flesh over metal muscles with no blood pressure in it. it's not human flesh. there's no such thing as "human flesh layer" - we have either muscles or fat/epidermis and so on. a terminator almost doesn't have those.
all that flesh is a completely artificial construction, though alive.
best example is the eye that T1 cuts out. it looks exactly like an eye on the outside... but it is just an empty half-sphere of biological material matching color of an eyeball. it doesn't work, and if it was a cloned eye, it would be an eye.
everything is connected and cross-dependant in an organism built by billions of years of evolution. if terminator's outer layer was a double of human construction, including aging, a terminator would have no place for endoskeleton.
for example: did you see any bone marrow in crashed terminator's "bones"? without bone marrow he cannot replace old/dead blood.
did you see any blood fountains? imagine somebody cut the eye & arm like T1 did or made a major arm cut like T2 did. he would loose all his blood in minutes. cyborgs didn't. no blood pressure => no function of blood => no NEED of that function because nothing common to nature products, everything's artificial.
and if skynet can crack the time/space mystery and made a liquid metal capable of copying live flesh (John's stepfather didn't notice anything wrong in his wife - imagine yourself standing next to a cold robot instead of a warm human with all those subtle odours), there's no way it couldn't produce alive matter from scratch and make it match human appearance.
aging is not a feature of flesh, it is a feature of a complete functioning organism, terminator's flesh doesn't fit those conditions => it's not a cloned organism, it's just shaped variants of material. of course it would weather over years - or may be even over
weeks, because there is actually no reason for skynet to make everlasting flesh for cyborgs who will be damaged after every bunker destroying, it's easier to replace the layer after every unnatural damage. but it would not age into an old man with hanging muscles which lost its tonus, because there are no muscles.
he would be either eternal (if the "outer layer organism" is fully self-sufficient) or very temporary (if skynet deliberately didn't give his terminators all our organs and glands supporting our immunity for decades until we gather enough damages from outer nature and our cells start failing to reproduce), but in any way it won't repeat human aging due to nothing common in complexity of human organisms (or any other organisms of our cycle - mammals).