According to this GameDaily piece, a Nielsen piece of research has shown that 17 percent of those buying GTA IV in the US were between the ages of seven and 16.
"61% of these younger gamers indicated that they purchased the M-rated game themselves, with 39% of the young gamers responding that someone else bought the game for them," Nielsen said.
"Interestingly enough, parents/guardians were pegged as the biggest facilitators for getting the controversial game into the hands of these young respondents, garnering 80% of the response. Friends, siblings and other relatives rounded out the other 20% of the response."
More through the link.