I think you need to look at how these figures are sourced (individual pitching coach - recording day 1 speeds). I would say the population is rather small and many of these day 1 speeds are recorded before any significant instruction on good pitching mechanics. Go back and look at the "goal" speed for these ages - quite a bit higher than these averages. I think the small population and how data gets captured (new student - day 1 pitch speed) may have something to do with the 14/15 sppeds not showing significant movement. If you are looking for averages, I'd go try to find something that taps into a much, much larger population. As for the older kids, I think the PG percentiles by class is a pretty good measuring stick.