Why is 8-years-old the age that "Mormons" are baptized?

Last week my youngest child announced that he will be baptized next year. What? I thought. No that can't be. But he just turned seven so he's right that next year he'll be baptized. The reason our children are baptized at the age of 8 is that Joseph Smith received a revelation from God that 8 is the age of accountability (Doctrine and Covenants 68:27). At that age, children know right from wrong and can make decisions for themselves. Therefore they can be held accountable for their decisions. We do not believe in infant baptism. Having studied some child development in college, I know that John Piaget (I actually looked this up just now to confirm it) named four stages of Cognitive Development in children. The third stage is Concrete Operational and develops between ages 7-11. Children are able to think logically. That seems to fit well with an age of accountability at 8.
If you are older than 8 and choose to be baptized, great! You repent of your sins and put your life in order so that you are ready to keep the commandments of God. Then when you are baptized, you are washed clean through the atonement of Jesus Christ. Jesus was baptized as an example to us that baptism is necessary to enter the kingdom of God. Because we make mistakes and are not perfect, we can repent and through the atonement of Jesus Christ, we can be forgiven. This is a continual process. We learn and grow throughout this life.
Now I have less than a year to teach my son everything he needs to know about the gospel. Not really. But we will have lessons as a family on what it means to be baptized so that he will be aware of the decision he is making. And then when his dad says the baptismal prayer and immerses him under the water, I will feel warm and happy inside knowing that my child has made a good choice.