By decay, I mean aging. The term aging is used with wrong connotations. For a lot of people it just means to get wrinkles, but in reality, it simply implies not reverting back to your original state. Believe it or not, human peak hearing age is when he is only 8 years old. After that, you lose your peak hearing ability. The point is, aging and decay does not mean getting crinkled, it implies that you simply keep biologically aging. For example, the skin elasticity and regeneration of kids under 10...you can never have that kind of skin, EVER. Which means, that once you enter the next stage, you have aged for that age group. I really cannot explain this better than this, I'm afraid.
Djinn is simply a word given for beings that are 'supposedly' - if used the orthodox religious logic- created from fire. As far as I have read, these demonic entities (Jinn is basically an arabic term for them) do not age at all. Unless killed by their own kind, I can't say they even die or age. If you age, sooner or later, you will die. That's why I cannot imagine how anyone would be able to control djinns. What people control is...I don't want to deviate the discussion, but it's Hamzad.