With the limited information you have provided, the offense appears to be either aggravated sexual assault of a child or indecency with a child. Presently, there is no limitations on when one of these offense can be initiated by the State. What controls, however, is what the law regarding limitations was at the time of the alleged offense and whether the limitations period has been changed over the past 10 years. The bottom line, however, is that the offense can almost certainly by prosecuted at this point. A person who is 12 at the time of an offense is juvenile and the offenses would have to be handled in juvenile court, even though the alleged perpetrator is now and adult. Consulting an attorney with experience in criminal and juvenile law would be a good idea as soon as possible.
Depending on the charge at the time the crime took place (the statute has changed) it could be 10 years after the victim turns 18 or no limitations. The alleged suspect can be prosecuted if the statute of limitations has not run.