Police investigating a prostitution ring questioned the Bayern Munich winger and Real Madrid striker for several hours at the Paris vice squad headquarters.
The offence is punishable by a maximum penalty of three years' imprisonment and a fine of 45,000 euros, although a conviction hinges on proving the accused knew the girl's age.
Ribery's lawyer said he did not know she was under 18 at the time.
The prosecutor's spokesman said Ribery and Benzema were released from custody pending further investigation after being questioned by magistrate Andre Dando.
Police had postponed the hearing until after the World Cup, in which Ribery was part of a disastrous French campaign that led to a public outcry after players protested against the coach and the team failed to win a single game. Benzema was not in the 23-man squad.
"One has to ask whether he is not being made to pay for France's World Cup failure," said Ribery's lawyer, Sophie Bottai.
French media have suggested the players were clients of a night club in Paris's Champs Elysees district that allegedly featured escorts, including the under-18 girl at the centre of the affair, Zahia Dehar.
The scandal, which gripped France before the World Cup, took a new twist in April when the young woman at the centre of the case gave an interview to Paris Match magazine, implicating players and denying being a prostitute.
She said Ribery, married with two children, had invited her to Munich in April 2009 to celebrate his 26th birthday in a luxury hotel, adding that he had paid her for three encounters last year.
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According to French law, paid sex with someone under 18 is deemed sex with a minor.
Zahia, who turned 18 in February, confirmed reports in the French press that she was under age when she met Ribery, but added that she had not told him how old she was.
Bottai told reporters that for her client to be found guilty, the girl "would have had to look like a minor and that was not the case and ... had informed him that she was under age, which was not the case".
Zahia had "said in all the press that she told him (Ribery) and others that she was an adult", Bottai added.
Ribery was France's most popular player before the World Cup fiasco, in which Les Bleus were eliminated in the first round without winning a match, and he took part in a revolt by players who boycotted a training session before their final game.
Police also placed Ribery's brother-in-law under investigation but released Benzema's agent after seven hours in custody over the incident.
Benzema, who was excused from training by his club for personal reasons, told French radio in April he had done nothing wrong. His lawyer Sylvain Cormier reiterated his client's position on television channel i-Tele earlier on Tuesday.
The "Zahia affair" will do little to improve the French squad's image after the team looked
