I recall an assembly program where an elder being interviewed said he was baptised at 6 years old.
There are examples reported in Watchtower literature of 6, 8, and 9 year olds being baptised. Here are a few of them....
Baptized at 6
(W 3-1-1992, p27. Pursuing a Goal Set at Six Years of Age, life story of Sandra Cowan)
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.
Baptized at 8
(yb02 p. 110 Curaçao)
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.
Baptized at 9
(w96 10/15 p. 32 "Jehovah Answered My Prayers!")
Consider, for example, a boy named Joel. He symbolized his dedication to Jehovah and was baptized at nine years of age.
Baptized in 4th grade (age 9) and 6th grade (age 11)
(w95 1/1 p. 24 “Out of the Mouth of Babes”)
During her six years of elementary school, Ayumi arranged for 13 Bible studies. She was baptized while in the fourth grade, and one of her friends with whom she had studied was baptized in the sixth grade.