At the Australian Royal Commission, Terrence O'Brien appearing for the JWs stated under oath that people are "baptised when they are of an age to qualify for baptism, so someone who is approaching adulthood or an adult.". See the transcript of day 259 Page 26539. He goes on to say that these ones would understand the implications of choosing to disassociate from the JWs, knowing the consequence to be shunning.
Of course this was argued in regards to those victims of abuse choosing to leave. But would a minor understand the implications of making a life choice to become a witness and that if ever they change their mind their own mother would never speak to them again?
But it is important to note that in order to qualify for baptism they say you must be of an age - nearly adult or adult.
I was baptised at 13. I never ever imagined I would find myself in the situation I am in now of having to hide my true thoughts and beliefs from family and friends so that they will still talk to me.