I studied ballet as a child and stopped for several years. Now that I am a young adult can I still become a professional dancer? 

Individual bodies and circumstances are different so there is no one simple answer. In general, continuous study from childhood through the teenage years is necessary to mold muscles, secure the necessary technique and strength, and acquire the movement qualities of a classical dancer. Previous training might allow someone to catch up more quickly in class upon their return to professional study. Other forms of dance, outside ballet, might also remain as possible options to pursue.
From NYCB’s Website

(Source: theballetblog)