Pretties by Scott Westerfeld
In the world of Uglies once you turn sixteen you also get to turn “pretty”. That is you undergo surgery that’ll get rid of all your old imperfections and instead make you beautiful according to the conventions of evolution and society. In book one Tally was the last of her friends to turn sixteen, and so was left alone when they all turned Pretty and moved to Pretty-town.
Spoiler warning for book one in effect.