The first section treats development of concepts concerning legal age from antiquity on and the evolution of the concept in Swiss law from the 19th century to the major reform in 1971. The Swiss situation is then compared to those in France, Belgium, Germany, Sweden, and Quebec. Data are presented on juvenile crime, the importance of the dark figure and factors distorting statistics, and the nature of delinquency in various age groups. The section on social prevention distingishes between juvenile delinquents and juveniles at risk and describes the relationships among penal, civil, and administrative measures, as well as criteria for choice of sanctions. Also considered are concepts of juvenile protection and the need to rethink legal provisions relating to juveniles. The author recommends setting the absolute legal age at 12, as most 12-year-olds are still living under the control of parents and commit minor offenses; developing multidisciplinary psychological and educative support organizations; and leaving problems of protecting minors with educational deficiencies or personality difficulties to civil authorities. Juvenile delinquents should also be guaranteed certain rights to protect against unlimited discretionary powers. Also proposed are abandoment of offender classifications leading to labeling and establishment of sanctions requiring minimal intervention. Recognition of the impossibility of always determining the juvenile's best interest is stressed. Notes and a bibliography are furnished. --in French.