As we know, the homunculi can heal themselves and don't age because of the philosopher's stone.
However, Wrath doesn't seem able to heal and he gets older, despite being a homunculus. I always thought that's because he is a human who was turned into a homunculus, but Greed can also regenerate, even though he still has Lings soul and isn't 100% homunculus.
Why isn't Wrath like a normal homunculus?
When Wrath discloses his nature to Mustang, he describes what happened after the incident. First, keep this quote of his in mind:
The Philosopher's Stone is created from the life force of countless human beings; it contains their souls.
Now, consider that Father was attempting to put all of his wrath within one being; this would require that he removed not only any wrath from within himself, but also every single wrathful soul that composed him. That means we now have a Philosopher's Stone made from dozens of wrathful souls. What does wrath do? It gets revenge.
Wrath states,
Countless souls have battled for dominance within me. And only the most wrathful one survived.
This means that he had no healing powers because he basically had no souls to regenerate from; only one soul remains within his body (his own or someone else's), and it is not enough to both regenerate and sustain him. He is basically a human with homunculus-level skills.
I also agree with Madara's conclusion that Father would want the public figure to age and appear human, so he may have been perfectly okay with Wrath being composed of only one soul. Pride's disguise was already conspicuous enough; you can imagine the uprisings if the military leader was also accused of such things.
With regard to why he is different from Greeling, his soul was pushed aside from the infusion, whereas Ling's was strong enough to coexist with Greed's.
I believe it was done on purpose.
Wrath was intended to be a public figure, everyone knows the king. So the king has to get old, or it would raise suspicion. The king can't generate wounds in a flash of red lightning, or it would raise suspicions.
How they did it, I don't know. But that's why I imagine they did it.
My theory has been that Regeneration costs a soul. Goes to explain why Hommunculi can be killed when 'killed' enough times, and why the mannequin soldiers don't regenerate.
As mentioned above, Wrath is the lone surviving soul within him, one soul, which killed all the others. The other Hommunculi have multiple souls cohabiting, which is why they're able to regenerate. The mannequins were likely made from a single philosopher's stone with the souls being split evenly between the soldiers.
Wrath (in Brotherhood) was a human-based Homonculus. He was born a human, called Fuhrer Candidate No. 12, and implanted with a philosopher's stone created from wrathful souls while in his prime. His own soul, also a soul of wrath, overpowered the souls of the stone and took over, making him Wrath, earning him the name King Bradley and the position of Fuhrer of Amestris.
it is because with Wrath, as Bradley states, all the souls basically fought down to the last one remaining. It was this final soul, of which was formed the homunculus "Wrath".
However, with Greeling:
First, the Homunculus "Greed", was already a pre-existing entity, which Father puts into Ling's body. Secondly, Ling accepts the pre-existing homunculus "Greed", without fighting down to the last souls, or last few souls. Thereby, as someone else had stated, Ling and Greed coexisted together.
