Here are the minimums that are expected of parish members (Catholics):
Cf. "Precepts of the Church," Catechism of the Catholic Church # 2242-43
Children are considered members with their parents until they turn 18 years old. Once they are 18, it is their own responsibility to attend church and to register as a member independent of their parents. At least one person in the family has to be Catholic (generally one of the parents) for a family to be parish members.