(part 2)

And, physically speaking, I was - so far as I can tell - not adversely affected.  That was 20 years ago.  I wasn't injured then, and haven't suffered an over-use injury since.  I've remained active ever since then (and, tellingly, set my marathon PR at 18, before I got old and fat while in undergrad).

Yes, all that "proves" is that my body at that time was ready to handle that level of mileage.  And I acknowledge that 13.1 miles at age 14 is very different from 26.2 miles at age 12, in some very significant ways.  But it may also be true that this young man is able to handle this mileage at this time.  I obviously hope that he is.

Finally, this article affords some interesting perspective:
http://www.nytimes.com/2009/10/27/sports/27marathon.html?_r=0