I may be wrong but if anything I'd suspect it's because children (talking about under-16s) are less likely to report. The article also suggests this: “We believe the figures show that those who commit these crimes target victims who are young and more vulnerable. Offenders have more confidence in getting away with crimes against younger women and girls."
I read this article in reference to the UK. Is their a world statistic as well? This is sad, but in my mind it is unexpected when I think about all the newspaper clips I read in the past year and with an understanding of when girls start in porn and prostitution, which is when they are very young.