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.
"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.
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
