Section 52. The following words as used in the following sections shall, except as otherwise specifically provided, have the following meanings:
''Court'', a division of the juvenile court department.
''Delinquent child'', a child between seven and 18 who violates any city ordinance or town by-law or who commits any offence against a law of the commonwealth.
''Probation officer'', a probation officer or assistant probation officer of the court having jurisdiction of the pending case.
''Punishment as is provided by law'', any sentence which may be imposed upon an adult by a justice of the district court or superior court.
''Youthful offender'', a person who is subject to an adult or juvenile sentence for having committed, while between the ages of fourteen and 18, an offense against a law of the commonwealth which, if he were an adult, would be punishable by imprisonment in the state prison, and (a) has previously been committed to the department of youth services, or (b) has committed an offense which involves the infliction or threat of serious bodily harm in violation of law, or (c) has committed a violation of paragraph (a), (c) or (d) of section ten or section ten E of chapter two hundred and sixty-nine; provided that, nothing in this clause shall allow for less than the imposition of the mandatory commitment periods provided in section fifty-eight of chapter one hundred and nineteen.