Okay to answer your questions in order of appearance.
My intention of him being a demon is multiparted. First, I would like him to be old enough to have been around before magic and technology had been around each other long enough to combine in their applications. As such I didn't know exactly how long ago it was when the worlds combined, so I decided 300 or so was old enough to be around before the merge of the two worlds especially considering you said the technology was about 21st century level. Also being a demon, he is a creature of magic and he would have such a long lifespan that any age applied to him would be almost pointless except to denote how long he has been around and how much he has learned. Second, I feel that playing a demon that is more human than his counterparts a novel idea. Too that end I would like him to be young enough to embrace the human ways as well as their technology.
Techno-mancy, my idea for this is fairly simple. I intend to use a melding of both systems, magic and technology. For magic things work by drawing arcane symbols, speaking an incantation, and occasionally making complex hand or body movements. Technology works by manipulating machines by: using code to tell what the machine should do (for most computer based technology: phones, cameras, modern cars, etc), while the other technology is mostly just an abstract combination of levers, gears and sprockets that when given the right stimulus,(ie electricity, water power, turning a crank or other types of levers) achieve the desired result of action.
The first form of technology described is the hardest to integrate with magic. But if you stop to think about it, the code that operates the machines is just a string of letters or symbols similar to that of an incantation. So you can replace this string of letters with an equivalent incantation that accomplishes the same thing. You can also alter (or hack) this code by using a incantation to modify the symbols contained in it allowing a person to change the purpose of a machine to his own designs. In order for this type of manipulation the person working the magic would by necessity need to know a lot about the technology in question. To help this a person could develop a minor spell that would tell the user in question about the inner workings of the device.
The second form of technology is a lot easier to integrate with magic as you can just replace the stimulus with magic or enchant it to release magic when given the right stimulus, like pulling the trigger on a gun to release a magic missile (if you play DND you know what this is, otherwise its just a small missile that is made of magical energy that shoots towards its target). By this you can string a bunch of simple machines together by using magic to accomplish larger effects and so on. In reality this is actually what more complicated machines are except they use code instead to integrate all the other parts of the system.
For a few illustrative examples imagine if you will, being able to hack a computer with waggle of the fingers and muttered incantation. Or to be able to type on the keyboard to make the table its resting on levitate. Able to animate electronic things so that they moved on their own, like a keyboard so that it can continue to type up a paper you were assigned or a post on a forum.
As to the wings, I don't really need them, they were merely a bit of artistic flair. That and I have a fondness to winged characters.
If I missed anything or you would like a little more detail on some area just let me know.