The teenager was charged with false imprisonment and fraud in the Düsseldorf youth court, which remained closed to the public.
Some 18 witnesses were called in the case, including the defendant's two victims.
Investigators freed the 13 and 15-year-old boys from the freezing Düsseldorf attic in October 2010. The defendant was 16 at the time the alleged crimes were committed.
The two children, who grew up in state institutions, were locked in the attic at night, forced to suffer temperatures well below freezing. By day the older teen forced them to steal for him.
The young man's parents were reportedly unaware of their son's criminal activities, which also included collecting donations for a fake charity.
Along with three other perpetrators the boy solicited money from businesses across the region for a non-existent school in Argentina, keeping the money for himself.
If convicted, the 17-year-old could spend several years in a juvenile correctional facility.