No one really knows where they came from. No god takes credit for creating them, nor does any sorcerer. For all intended purposes, one moment there were no demons, the next the were scattered about Faerune. Their race was already unique, as they have a vast array of appearances between their, their culture already developed, their wars and their pacts already established. It was as if they had a hidden society from all, even the gods, then something happened to make us see them, hear them, feel them.
Since then, they have adapted to our world without blinking an eye. They have mostly mixed in, as humans have done, with the blood of other races, though there are always the pockets of pure-blooded demons.
Demons deserve a special note for their appearances, as they are so widely varied ; more so than any other race in Faerune. The standard conception of the appearance of a demon can still apply, but many have taken to hiding int he skin of humans or other humanoids for one reason or another. In truth most races' images could be plausibly be used to represent a demon.
Damn near everywhere, depending on how your demon is built.
No more than 1000 years old.
Herb-,omni-,carni-, and in some cases everything-vore
Anything that can cross breed can breed with a demon.