Zatopek, reading your excellent posts induced an out of body experience for me. I agreed with them so thoroughly I thought I might have created your account and written them in my sleep. (I happen also to be a lifelong Zatopek aficionado!).
I've actually coached a number of very good high school girls over the past few years, including the MacDougall sisters, Branna and Brogan. I have repeatedly emphasized exactly what you argue-- that progression should be carefully timed through the watershed ages of 14-15, and that doing so does not diminish the athlete's enjoyment of the sport even in the short term.
The MacDougall girls were both very good on deliberately minimal training in 8th grade (around 5:00 for 1500). It was obvious to anyone (except maybe themselves) at that time, however, that they could have run much, much faster on more serious training and with more frequent and higher level racing opportunities. Since they had no idea of their potential, they simply enjoyed being competitive and looked forward to reaching the next level. As a result, both have managed to progress at or near the average rate into and through their middle teens, in spite of being quite fast in their very early years. After a very good but far from earth-shattering high school career, Branna ran 15:48 (summer after graduation), and Brogan, now a high school junior, has started her season with a 16:08. To reinforce your point, I am completely convinced they could have run these times much earlier. The trouble is, they might have been telescoping their progression in doing so, and then possibly had to endure 2-3 years of flat performances-- and that's if they'd had the desire to stick with it at all. The future would look completely different to them today if they had run 16:00 at age 15 and were still at that level-- even though, objectively, they'd be the same runners.
Keep up the good work. Heartening to know for certain that there is at least one other youth coach out there who is trying to think it through, rather than take the line of least resistance!