What does it mean if someone underage that acts like she never would, starts smoking?

It means she is still figuring out who she is and wants to try new (and possibly rebellious) things.  She either is hanging with other smokers and wants to be part of the crowd, or perhaps she is hanging with people who don't smoke and wants to stand out from the crowd. Another possibility is that she wants to emulate a smoker she admires.  In any case, hopefully it won't take her long figure out she made a bad decision.