Changelings were and still are human in a sense, they have vastly longer life spans than humans and are
treated as nothing more than slaves within Allutheria and in the human world they are shunned for their difference. Stolen from one world and not fitting in within either one, that is their unfortunately lot in the world.
The Goblin King presides over them and creates them through a series of spells he keeps carefully guarded from others, he will never share the secret of how they are made. Perhaps only one other person knows it and even then he only knows parts.
Changelings came about shortly after Jareth took over the ruined Court of Bull and began to reign over it as the Goblin Court. He created them out of a need to power torcs, a special magical device the Goblin King designed in order to provide the goblins he ruled over with magical abilities.
It took years of trial and error and many human children died before Jareth succeed in creating the first true changeling. This changeling quickly became Jareth's favourite and would be one he would never give up or sel to anyone. If there is any changeling within the labyrinth that holds rank, it would be this first changeling.
The life of a changeling is not easy, most as stolen from their homes as babes, snatched from cribs by goblins or by the King himself. Once brought back into Allutheria, the process used to change them... well it is less than pleasant and Jareth hardly cares about the state the human children live in, as long as they survive. Though once he has stolen their years from them through a series of spells, he has little care for what happens to them. He has what he needs at that point.
It was not until a noble from one of the other courts saw him traveling with his favourite changeling and promptly offered precious gems and coins for the creature that Jareth realized they had yet another purpose. Much like the goblins, they were not only good for one thing.
Seizing the opportunity the Goblin King began selling changelings for a high price and they quickly became a valued commodity.
Changelings are the lowest of the low within the goblin court. They are considered beneath the goblins and beneath the fae alike.
Within the changeling's there is an unspoken rule, Jareth always has a favourite and it's best to be his favourite. The way the goblin king views the changelings is as less than nothing. Occasionally he is able to sell them off to other fae and then they are worth the price someone is willing to pay.
Changelings look exactly like humans, they remain a certain age, however. It all depends on how far along in the process they are and on Jareth's whims when it comes to how old he wants them to look. They will eventually age at about the rate of a fae. Jareth had wanted to recreate the fae's life span. The magic is not perfect, but he is still working out the kinks in it.
Jareth originally started collecting the human children in order to use them as batteries to fuel the torcs he provided the goblins with as a source of magic. The years he ages the changelings, the years they loose, become the power for the torcs. The process does not mentally age the changelings, only physically so when they are taken to the goblin court they become children stuck in the bodies of adults.
Changelings are by nature still human, they have simply been magically changed in order to extend their life spans and keep them locked at a certain age. They are capable of reproducing, however it is frowned upon within the Goblin Court.
The children that a changeling might produce are human and lack any magical nature that their parents would have.
Changelings have all of the abilities of humans and no magic to speak of. They are created as tools and subjugated to a life of slavery. They're not seen as anything other than useful creatures meant to be stepped on by the fae.
Changelings have all the weaknesses of humans, aside from their significantly lengthened life spans.