How was the definition (age) of the child decided?

As mentioned earlier, by confining the Act to Article 21A. The government’s decision to do so has obviously got Parliamentary approval with this Act. The original Article 45, and the Unnikrishnan verdict both include the age group 0-6. The Juvenile Justice Act defines a child up to age 18. The United Nation’s Child Rights Convention (UNCRC), to which India is a signatory, also defines a child from 0 to 18. In principle, by referring to the JJ Act, the UNCRC and Article 21 (right to life) in the Aims and Objects of the RtE Act, the age could have been defined from 0-18. However, citing economic compulsions, the present Act has been confined to the age group 6-14 as contained in Article 21A. A great deal of public pressure would need to be kept up in order to have the Act amended to incorporate the 0-18 as the age of the child.