Is it illegal in Illinois for 2 15 year olds to have consensual sex?

Yes. This is illegal.

The legal age of consent to have sexual contact is 17. The two 15 year olds would likely not be held accountable in the criminal justice system, but rather the juvenile justice system. This would be a matter presented before a family court judge. The laws of the adult criminal system of justice apply to all citizens even juveniles; however, juveniles are held accountable to these laws based on the standards of the "Juvenile Justice System" where the emphasis is on rehabilitation. Criminal liability is not the same.

Juvenile cases can be referred by a family court judge to the District Attorney to be heard in adult court but this is usually in cases involving violent crimes and/or habitual offenders.