The Shadow world has technically existed longer than the human world, but the Shadow society as it is known today has evolved alongside human society. The Shadow world has twice as many inhabitants than that of the human world, but despite this, the society only manages to progress at the same speed as that of the humans'. This is due to the fact that Shadows are not in charge of their own independant society, but depend on the workings of the human society to function correctly.
Shadows do not fully understand the concept of gender, as it is a human thing. Before humans existed as they do today, Shadows were nothing more than figments created with the energy left over from the birth of living creatures in the realm opposite the Shadows' own. They had no idea what gender was, and no necessity to impliment such an identification system. As humans evolved, however, a clear distinction between sexes emerged. Along with this came the idea of differential gender identities. As Shadows began to hunt human for their physical attributes, they began to adapt the concept of gender into their everyday lives.
How Shadow hunting works is simple: when in their original Shadow form, they are undetectible to a human being. In this state, they can enter the body of their prey and contort their organs, killing them in a way that seems natural to any autopsy or other medical procedure given post-mortem. The usual way that Shadows kill their prey is via cardiac arrest, as it is the easiest to excuse in most cases. The spoils that Shadows receive from hunts are the physical attributes of their prey, as well as how much time the prey had left to live in their natural lifespan.
Once back in their city, a hunter Shadow can redeem their spoils at the headquarters of the guild they belong to. The hunter receives first pick of the physical attributes - as they might have killed the human to receive a trait for themself - as well as half the timespan received. The rest of the timespan is distributed amongst other members of the guild (with half of the remaining span going to the leader and the remaining quarter being divided equally among remaining members), and the remaining physical attributes are sold for currency or more timespan in the store owned by the guild.
Shadow hunts are usually only carried out on target based necessity, usually conducted when the city the guild resides in is short on a resource or in need of the newest trending looks. Shadows, though they are a shape-shifting species, cannot shapeshift unless they possess the look they would like to appear as. This is why hunting for physical attributes is necessary. As far as timespan goes, a Shadow can live indefinitely if it harvests lifespan from other living beings. In the beginning of hunting, the method for harvesting spoils was crude. Physical attributes could only be collected for a long while, causing elder Shadows to die out within a few millenia. In the modern day, it's expected for an average Shadow to live at least 5,000 years. For a Shadow to cease existing before this milestone would be considered premature death.