Since January 2014 the French national identity card is valid for 15 years, as opposed to 10 years previously. Cards provided before this date benefit from an extension of 5 years to your validity. However, there is no extension for people who were less than 18 years old when they created their ID card.
Specific information for minors travelling alone on flights outside of French territory: From 15 th of January 2017, minors travelling alone must present a form of authorisation to leave the country (AST - autorisation de sortie du territoire) signed by the holder of parental custody, for all international flights including those to destinations within the European Union. It must be accompanied by a copy of the identification of the person who signed the AST
NOTE: Ryanair does not accept unaccompagnied minors under 16 years old.
For more information we invite you to consult the site of the French Ministry of Foreign Affairs and International Development.