Since January 1999, all young French men and women must register at their local Town Hall within 3 months of their 16th birthday, and then take part in the Defence and Citizenship Day (JDC). 

If these steps are not taken, it is not possible to register for state examinations (driving licence, baccalauréat, etc.) before the age of 25.

More information : https://www.defense.gouv.fr/sga/au-service-nation-du-public/jeunesse/devenir-citoyen/recensement