That six year old number is not speculation. There are no less than three Watchtower publication examples.
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. The Watchtower, 1992 3/1 p. 27
In 1934, Mom and Dad were baptized. I too wanted to get baptized, and I kept insisting until Mother asked an older Witness to talk with me about it. He asked many questions in a manner that I could understand. Then he told my parents that I should not be prevented from being baptized; it might harm my spiritual growth. So I was baptized the following summer, when I was still six. The Watchtower, 1996, 8/1, p. 21
A survey of the publishers in Ghana showed that 12 percent of the more than 50,000 publishers were between 6 and 20 years of age. Yearbook, 1998, p. 14
Len Miller