Gilded Cage by Vic James

In a darkly fantastical debut set in modern-day Britain, magic users control everything: wealth, politics, power—and you. If you’re not one of the ultimate one-percenters—the magical elite—you owe them ten years of service. Do those years when you’re old, and you’ll never get through them. Do them young, and you’ll never get over them.
 
This is the darkly decadent world of Gilded Cage. In its glittering milieu move the all-powerful Jardines and the everyday Hadleys. The families have only one thing in common: Each has three children. But their destinies entwine when one family enters the service of the other. They will all discover whether any magic is more powerful than the human spirit.
 
Have a quick ten years. . . .