In Uglies, coming of age isn't just about going through puberty and having to take the SAT. Coming of age means being able to find a place in the world: a home where you belong, some work that's important, a goal or idea that motivates you. (Also important to find: a comfy restaurant you can go to in your pajamas.) The pretty surgery is a classic rite of passage that distinguishes the ugly kids from the adults; but in this world, we learn that the adults may not be all that mature. So it's up to Tally and other young people (pre-surgery) to fix the system and make the sort of world they would want to live in.
Characters in Uglies come of age when they learn what matters most to them (sometimes hoverboards, sometimes relationships).
Coming of age in Uglies has nothing to do with age and everything to do with friendships and identity. Tally comes of age when she realizes who she is and what friendships mean to her.