from the article:

"Though Estes & Weiner didn’t share their data the charts provided by the anti-prostitute organization Shared Hope International (which seems to be the original source of the distortion from “the average underage prostitute” to “the average prostitute”) demonstrate (as Emi calculates) an average of 15.91, three years higher than their verbal claims and slightly higher than the average reported by the John Jay study, which was 15.15 for girls and 15.28 for boys. Incidentally, it also found that only 15% of teen hookers entered the trade at an age below 13, which absolutely demolishes any notion of 13 as “average”."

With the caveat that the Jay study covered only New York City and the SHI study covered 10 cities in the States, it would appear from the statistics they gathered that 13 is an inaccurate number for the US, with the 15-16 age range being the correct one. The University of Pennsylvania sampled from the US, Mexico and Canada; there would have to be a massive amount of extremely young children being trafficked in Canada and Mexico to skew the age range that far, so I don't know what they found, or where, to give them that number. Links to all the reports are there if you want to read them.