Sin knows no age whether it is an infant, or old man, all men are sinners from birth. Thankfully, the Lord God is merciful in that he "only" holds man accountable for what he know between right, and wrong. The more he knows of what is right, the more responsible, he is held accountable to do it.
As it is written, ".....unto whom much is given, much is required..." (Luke 12:48)
A person -- whatever age -- needs to be able to communicate with another person in a verbal way. To be able to understand when salvation is explained to 'them' and then be able to respond Back.
A baby's language is crying -- hunger -- needing a diaper changed -- pain. And a care-taker would take care of those needs. A child / adult needs to be able to understand what another person is sharing with them and then respond back verbally or written. That is what is referred to as being the 'age of accountability'.