Although extremely hard to procure, it has been shown that young Demons are far more Magica than they are flesh.

Imagine a delicate and ripe fruit. Although it gives the appearance of solidity, if you squeeze it will yield completely.

As a Demon ages it is able to compact and refine its own energy into an increasingly dense structure. 

Still, even solidified Magica is still Magica, and in turn it is weak to disruption by Magica. 

The purity of Magica needed does increase in relation to the refinement of the Demonic form.

-Notes on "Demonic Flesh; Methods for Efficient Vivisection" by the Head Scientist Priest