a young person who shows a tendency to commit crimes: teenage delinquents
a person, usually young, who behaves in a way that is illegal or not acceptable to most people:
Someone, usually a young person, who is delinquent repeatedly commits minor crimes.
...remand homes for delinquent children.
Synonyms: unruly, wild, rebellious, anarchic
Delinquent is also a noun.
...a nine-year-old delinquent.
behaving in a way that is criminal or immoral
Delinquent needn't mean criminal, although it often does. Collins gives 'unruly, wild, rebellious' as synonyms. I would guess the author wants to paint Mary Austin as a bit of a wild child.