Satan has been around for 17 years, after being formed from Lucifer's wrath. The magic of Lucifer's torn wings and anger were enough to manifest another being to contain the sheer amount of fury: him. Satan isn't a demon, even though he has the form of one, and has to keep his wrath in control, or he may end up cracking and breaking apart.

That is why he is currently curled on the bathroom floor, having argued with Lucifer, and his overflowing rage forming bleeding cracks in his being.