This varies from denomination to denomination. It’s typically when you’re baptized as a Christian. For my denomination - the Church of Jesus Christ of Latter-day Saints - the age of accountability is eight years old. This gives us time to learn right from wrong, make mistakes, etc.
before it’s all heaped onto our shoulders as something we must repent of. At baptism, our sins are washed away and we are “born again” of the water. After that, we have to start repenting of our sins. This takes a great vigilance that is learned over time.
However, I cannot speak for other denominations of Christianity. For more information from a different denomination, you’ll have to ask a member of that denomination for their beliefs.