Carbons and System units such as Sera and Trigger are VERY similar. They are both artificial beings- and both seem to be able to age. Although I cant entirely be sure about the Mother Units such as Sera and Yuna themselves, since they are a whole different class to Trigger. Or Juno for that matter. We cant say what units are similar to Carbons and which are more different, sicne we know little about the Master System and it's robots, aside from a little of it's structure. (Beurocratic models, 2 Mother units which monitor Earth and Elysium respectively, Purifier units which hunt down aberrant units that go against the system- named "Irregular Hunters" in Japan if I recall-)
But again, they were also able to trick Barrel and Von Bleucher that they were "Ancients", and Sera DID need to rest to recuperate after Rock recovered them, so they must be pretty similar.
But just think like this. Rock = Trigger, = a former system unit. He is indistinguishable from a carbon, although the fact that he probably looks different to how he used to is a factor there. (armoring, etc) He can replace his body with a robotic digger body, as folks can do that. Some opt for smaller changes, like an arm or so, and others swap out the whole body.
Think about the ZX world, and how Reploids are much harder to differentiate from Humans aside from forehead triangles. Now imagine Carbons as the final step in that evolution. Artificial Humans who are very much like the real thing, and suffer the same ailments.