The question of when one has reached the age whereby he/she is held accountable for their sins is one that has had very little clarity throughout church history. Some say it is the age of seven. Others, taking a more careful and thoughtful approach, suggest that age may be different for different children. For example, a child who has learning difficulties or other issues impacting emotional and mental growth will be older before they reach the age of accountability, of ever they do, depending on the type and extent of difficulty or condition. Meanwhile, some children mature faster, and so may reach the age before their peers. While I certainly take this approach myself, I also have to recognize what is normative in Apostolic Tradition. In researching this topic, I found only one statement from an Ante-Nicene Father, which is as follows:
"In pursuance of that aspect of the association of body and soul that we now have to consider, we maintain that the puberty of the soul coincides with that of the body. Generally speaking, they both attain together this full growth at about the fourteenth year of life. The soul attains it by the suggestion of the senses, and the body attains it by the growth of the bodily members. I do not mention this age because reflection begins at this age. Nor do I choose it because the civil laws date the commencement of the real business of life from this age. Rather, I choose it because this was the appointed order from the very first. For after their obtaining knowledge of good and evil, Adam and Eve felt that they must cover their nakedness. Likewise, we profess to have the same discernment of good and evil from the time that we experience the same sensation of shame. Now, beginning with the aforementioned age, sex is suffused and clothed with a special sensibility. This eye gives way to lust and communicates its pleasure to another. It understands the natural relations between male and female, and it wears the fig leaf apron to cover the shame that it still excites."- Tertullian
Essentially, Tertullian argues that, once a child has an understanding of nakedness, and of human sexuality, that child has reached the age of accountability. In his culture and time, that age was fourteen years old. In today's cultural climate, it is usually much sooner than that for most children. So here we have an Apostolic guideline: once a child has an understanding of the shame of open nakedness, and of human sexuality, that child has reached the age of accountability. In our current culture, that age may be anywhere from six to eight years of age, if we are not dealing with extraordinary circumstances. Whatever the case, however, fourteen years of age is the basic guideline and rule of the early church.