I think TCFs claim that many top boys juniors leave ITF jr competition age 14-16 and start on the pro tour that young age, playing Futures/Challengers, is not correct. There are very few of them, and usually they mix it up with futures and Junior tournaments....so they have junior ranking as well.
However, for girls it is more like that, some of the best jump to the pro tour early and leave ITF juniors completely at age 16. Still most of them mix it up and at least play jr GS, but not all.
Girls mature earlier and I think there also are rules on max pro tournaments for certain ages that strengthens this difference.