The boy, identified only as "B", was 16 when he left with three others from the town of Nice in southern France on December 27, 2013, giving no warning to his family.
A recent convert to Islam, he took a plane to Turkey before continuing by land to Syria. His mother, who spoke to him recently by phone, says he is still there.
The family's lawyer, Samia Maktouf, said Turkey was "notorious as the route to enter Syria".
The mother is seeking damages of 110,000 euros (USD 120,000) from the government for her and her three other children.
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"It's not the money that we are interested in, but we want it known that an error was made. The departure of minors for jihad must stop," Maktouf said.
In France, teens can have sex at 15 but only get a vote in national elections at 18.
The boy's mother was not present in court because she was "devastated" by the news, received a few days ago, that one of the youths who travelled with her son to Syria had been killed, her lawyer said.
"He was not subject to a ban on leaving the territory (and) there was no search warrant for him," said the government's counsel in court, calling for the case to be thrown out.
"Turkey may be an entry route into Syria, but it is also a holiday destination," he added.
"Nothing in the behaviour of this adolescent showed that he was trying to leave the territory without the permission of his mother."
Several minors were stopped last year trying to make the journey, but usually as a result of information from their families.
The government has recently passed laws allowing it to confiscate identity cards and passports from those suspected of seeking to join the jihad in Syria.