The defendant, named only as Mertkan G., was arrested in October 2014 when he was 14 and allegedly had contact with supporters of the Islamic State (IS) jihadist group and Al-Qaeda, the court in his hometown Sankt-Poelten said.
"In addition he is suspected of obtaining instructions on how to make an explosive device from an Al-Qaeda website... In order to carry out an attack in Vienna," the court, 70 kilometres (45 miles) west of the Austrian capital, said.
Police had said at the time of the arrest that the boy made "concrete enquiries about buying ingredients" for a bomb and "planned to explode the devices in public places, such as the Vienna Westbahnhof," a major train station.
His lawyer Rudolf Mayer told AFP that his client had only been "playing with the idea" of making a bomb. He has been in custody since January after breaking the terms of his parole.
In common with other European countries, Austria has seen a steady flow of people leaving or attempting to leave the country in order to join IS militants in Syria and Iraq.
According to the Austrian interior ministry, more than 200 have done so, including some women and minors. Around 70 have since returned, several of whom are in custody awaiting trial.