Watchtower has published articles detailing the baptism of children as young as 6 years old. Two examples..
(Baptized at 6)
In the summer of 1946, I was baptized at the international convention in Cleveland, Ohio. Although I was only six years of age, I was determined to fulfill my dedication to Jehovah. That summer I served as a pioneer for the first time. I remember one morning placing 40 magazines with people seated at The Plaza in San Diego, California. My being tiny and talkative had a lot to do with it, I’m sure. (W 3-1-1992, p27. Pursuing a Goal Set at Six Years of Age, life story of Sandra Cowan)
(Baptized at 8)
"In Bonaire, six-year-old Renzo was invited to the Kingdom Hall and enjoyed it very much. A Bible study was started with him, and from then on he refused to go to the Catholic church....Subsequently, Renzo’s father and mother along with one of Renzo’s Bible students were baptized. Renzo, now eight years old, was baptized at a circuit assembly in Bonaire." (Year Book 2002, p. 110 Curaçao)
Only adults were baptized in the Bible. Even Jesus himself, who was no doubt much more advanced than normal 12 year olds and was discussing scripture with the priests, was not baptized until he reached adulthood. When he was 12 the Bible says - "47 Everyone who heard him was amazed at his understanding and his answers." (Luke 21:47)
Watchtower has continually said that 30 was the age of manhood in that society, which was Jesus' age at the time of his baptism.
Baptizing small children is a clear example of "going beyond what is written."
As a side point, when those baptized children are misbehaving in some way they are treated as adults in judicial situations.
"When a baptized minor becomes involved in wrongdoing that threatens the cleanness of the congregation, the assigned committee should meet with such individual just as they would with any other member of the congregation" (Pay Attention to Yourself and to All the Flock, p. 98)