This is a question many parents ask. In the Bible, there are several examples and options for the age of accountability. The most common, considered by church communities, is age twelve. This was the Jewish age of Bar Mitzvah and Bas Mitzvah considered the age of adulthood. However, when God cursed the Israelites who refused to enter the Promised Land because of the fear of giants, He said all who were over the age of twenty would die in the wilderness. So is twenty the age of accountability? Studies show that the greatest percentage of people who give their lives to Jesus is much younger than that and the chances of making that choice drop drastically after that age. Once a life pattern is set it is very hard to break and happens rarely.
There is, however, another example in the Old Testament where God judged extremely wicked cultures and held accountable those who knew their right hand from their left. This occurs somewhere between ages three and five. I know a great many life long servants of Jesus who asked Him into their hearts at three to five years of age. I personally did it at age three and remember it clearly. So how do you prepare your child for this?
Start reading the Bible to them while they are in utero. They can hear you and their spirits will understand even though their minds are underdeveloped.
Read Bible stories to them from birth onward. Help them to know God as Creator and loving Father. Add stories of children doing something wrong or unwisely and needing to confess and rebuild fellowship with someone.
Watch and listen to questions they ask and things they tell other children. You will realize when it is time to tell the story of Jesus' salvation for them. Always make Easter more about Jesus' payment for how naughty we all are than about bunnies and Easter eggs.
In my book "The Four Seasons of Fruitful Parenting" I have an easily drawn visual that my mother used to lead thousands of children, ages four through twelve, to Jesus. Many of these are pastors and missionaries today. This is simple, clear, easily understood by young children and all explained in my book. The hard work is done for you.
Remember some children may want to ask Jesus to save them several different times. Go ahead and pray with them again. They may be simply growing in greater understanding of what Jesus has done for us. But also explain I John 1:9 that once we are in Jesus' family we just need to get a few naughty things right with Him. Jesus said to Peter in John 13:10 "He who has bathed needs only to wash his feet, but is completely clean;" When we have a bath we are completely clean but running around can get our feet dirty. We don't need another bath just a washcloth for our feet. They will understand this concept.
Never tell a child that they already asked Jesus to be their Savior. Let them tell you when they did because that was the time when their heart was fully reached.
There are teens and older ones who have been told that they are saved because they copied someone in saying a salvation prayer but their heart never connected with repentance or Jesus' ownership. These are like people who are vaccinated. They were given a watered-down version of the disease so they wouldn't actually fully get the disease. These have become vaccinated against true Christianity and true ownership of Jesus Christ and may never join Him from their hearts.
So start young but be careful of assumptions. Watch for changed attitudes and actions to know that Jesus is in there doing His work. Even a little child will give evidence of true salvation.