the age of accountability may differ by denomination, but having said that, the second one says "by denomination" one has just pointed out the issue of social peer pressure. many children fall into wanting to please this social group of family and denominational group and prematurely get baptized. others fall into a system of courses that culminates into a baptism, whether the child is ready or not at the end of that system of courses is irrelevant, because the baptism gets done as it is the end of this system of courses, therein it maybe be done prematurely. others are baptized at birth and unwittingly have been baptized without making any conscious decision of its implications.
for a baptism to be effective, the decision must be a mature and personal decision without pressure but rather a calling, and after one understands the meaning, the reason, and the responsibility thereafter, then it is a good time to be baptized. that age differs from person to person and the social environment in which they are brought up rather than which denomination they are.