Was Shogun artificially aged? I got the feeling that he had to be because Nanami is his real sister and she is not that old. Also the nurse "Grim" was performing some king of operation on him. Before that operation he appeared to be young as he lacked all of the wrinkles on his face. Also if he was aged how was that done? What was the yellow goop that "Grim" scooped out with the weird spoon?
As seen in the anime (carefull this contains spoilers).
Towards the ending shogun reveals that he actual is Takumi Nishijo His original. and the main protagonist was just a fantasy of him. In this part he also reveals that he has a rare disease which causes him to age when he uses his power (believe this information is given spread over episodes 10-12). They do not go into details about how he got this disease
With this the answer to your question would be No, Shogun was not artificialy aged.