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It took some digging but the 12-17 claim was often pointing to one website which was simply making an undocumented claim. Eventually I was lead to the "3rd Way," a "nonpartisan" progressives website and to a PDF which has a footnote for the same statistic. Here's the footnote:

Mark B. Kastleman, The Drug of the New Millennium: The Science of How Internet Pornography
Radically Alters the Human Brain and Body, 2nd edition, Granite Publishing, Orem (UT), 2001;
see also, http://internet-filter-review.toptenreviews.com/internet-pornography-statistics.html,
accessed on: July 18, 2001; Bella English, "The Secret Life of Boys: Pornography Is a Mouse
Click Away, and Kids Are Being Exposed to It in Ever-Increasing Numbers," The Boston Globe,
May 12, 2005; Crystal Roberts, Internet Filtering and Blocking Technology: The Most Effective
Methods of Protecting Children from Pornography, Family Research Council, Washington (DC):
September 3, 1999, available at http://www.copacommission.org/papers/ is99g2pn.pdf, accessed
on: July 15, 2005

Following that footnote took me to the COPA commission document and, again the 12-17 "one of the largest groups" and another footnote. That citation is for "Final Report of the Attorney General's Commission on Pornography (1986)" YES: 1986.

It seems we may have a claim about 12-17 year olds and uses of Internet pornography being derived from a 1986 report! Oops?

Fascinating.