Section 2(12) of the JJ Act, 2015 defines 'child' as a person who has not completed 18 years of age. Sections 2(12), 2(13), and 2(35) together clarify that a child or juvenile is anyone under 18, and a child in conflict with law is one who commits an offence before completing 18 years. The Act also introduced categories of petty, serious, and heinous offences — a departure from the earlier 2000 legislation.