http://www.usatoday.com/news/nation/2008-09-29-dogfighting_N.htm
"The most active and numerous dogfighters, experts say, are 13 or 14 or 17 years old "
""There are at least 100,000 young kids fighting their dogs under the radar in America," estimates Chicago-based anti-violence advocate Tio Hardiman, who built his estimate on conversations with young dogfighters and authorities in 35 states he has visited. In contrast, about 40,000 adults are involved in organized dogfighting, according to the Humane Society of the United States."
" "I saw a kid in a park, he was probably 8 or 9 years old, training and strengthening his pit bull by having him tread water in a creek.""
"It actually generated more interest among urban youth," Grim says. Suddenly, kids who had believed dogfighting was only a ghetto or rural Southern sport saw rich role models were involved. "They thought, if (Vick) does it, it's cool."
"We discovered (in St. Louis) a group that held a dogfight for a church fundraiser, and that sends a pretty strong message to children,"
Wasn't the article I was looking for, but worth quoting anyway, I think.
ETA: Not sure this is from a "reliable source" but is very recent (about a month ago)
"Sandra Bullock's husband
Jesse James is involved in another scandal as he is accused of animal cruelty. The West Coast Choppers CEO allegedly took his pit bulls to one of those dog fighting rings and forced the poor animals to participate in a battle for their lives"