I haven't noticed the very top pitchers showing up at NFCA camps. I don't suppose they need the exposure. Possibly even committed already. I'm thinking their average speeds would skew these results upward at the older age groups.
While your point is valid, I would think any given year the up and coming pitchers would balance out the statistics. Junior and seniors may not be going but I bet there are a lot of freshman and sophomores going looking for exposure.
A couple of years back I compiled the statistics from all NFCA camps, just over 21% of the total pitchers reached 60mph. Some camps had no pitchers over 60. The largest number of girls over 60 was the NFCA camp from the Colorado 4th of July tournaments.