Why young athletes slow down during puberty, and many tap out soon after

Young athletes end their involvement in sport for two reasons: because it's no longer enjoyable, and/ or because they stopped making progress a while ago.
Many young athletes experience a performance plateau during puberty.  Speed/ endurance athletes often get slower.  The body grows significantly before puberty, and power to weight rises.  This, more than any other factor - coaching included! - is responsible for progress.  With the onset of puberty the nature of weight gain changes.  For females, the extra weight is non-lean mass (not muscle) due to the expression of the hormone oestrogen.  Before puberty, weight gain is associated with linear increases in strength (and power to weight).  With the onset of puberty, weight gain now has the opposite effect - it increases body weight for no additional gain in strength. 
There is a secondary factor superimposing all of this, which is that sporting skills tend to mature around the time of puberty as well.  This is a natural consequence of practice over time.  The problem is that the brain and body are running out of resources or tools with which to keep things moving forward: the body is now much heavier but not stronger with it, and the brain is also finding it harder and harder to refine skills.  We call this diminishing returns: more and more training time for less and less reward.
This is all very normal and impacts the majority of female athletes and some boys as well.  It also doesn't have to be the end.  What's needed, in the first place, is an explanation to them that this is what's happening.  In the absence of an understanding, children think it's their fault; they're just not good enough any more.  This interpretation couldn't be further from the truth.  New rules now apply and these need to be articulated and understood by the athlete as well.  There are there things children need to know:

1. what happens to my body and my performance, as a consequence
2. what do the changes mean - what are the new rules
3. I will be the best I can ever be another decade and a half further down the track (or more) - I have loads of time to get used to, and take advantage of, the new rules

Parents it's your responsibility to understand all of this and play a role in communicating to your child.  It's also critical that coaches understand, and true as well that most don't.  Hold them to account and insist they get off their butts and go find out.  It will likely change their coaching at every level, and if that means they stop treating every child like a performance athlete and do a better job of making sport more enjoyable then that's the biggest win of all!