It was a mystery as to where the young girl had acquired such a cynical attitude.

Cynical : believing that people are motivated purely by self-interest; distrustful of human sincerity or integrity.
Mature : fully developed physically; full-grown.
Naive : (of a person or action) showing a lack of experience, wisdom, or judgement.
Eccentric : (of a person or their behaviour) unconventional and slightly strange.
Crazy : mad, especially as manifested in wild or aggressive behaviour.

Antonym of Cynical is Naive