There is no single administrative definition of 'youth' in France.

Without having strictly defined limits, youth policies generally concern those under 30 years of age. Child protection measures apply up to the age of 18, which is the age of majority.

This period of life is marked both historically by the end of a legal obligation, that of the end of compulsory education (being 'at school') at the age of 16. Training (the acquisition of new skills in formal and non-formal learning environments) remains compulsory until the age of 18 since September 2020.

Source on mandatory education until 16 years-old: article L131-1 of the code of education (code de l’éducation).

Source on mandatory training until 18 years-old: law n° 2019-791 of July 26^th, 2019 for creating and supporting trust within a school environment (loi pour une école de la confiance)

This period of life is also marked by new responsibilities:

The age of criminal majority is, in France, set at 18 according to the Code for juvenile criminal justice (Code de la justice pénale des mineurs). A young person can incur criminal responsibility from the age of 13, but by becoming a criminal adult, the young person can no longer reduce their sentence by benefiting from the excuse of minority due to being considered an adult as the age of 18.

Being 18 also provides with the possibility of exercing new rights (e.g. voting) and benefiting from certain social benefits, such as the active solidarity income (of which there is a "young" version before the age of 25).