Usher's comments are dead on right. Burnout is so much more of a mental phenomenon. Yes it's easier to stay motivated however when you are improving greatly due to increasing your workload and those that train very hard sooner may discover a plateau at a young age while others around them are still enjoying the upswing. How you handle that plateau and maintain your drive and enthusiasm and optimism will determine if you continue to more success and improvement or become disenchanted and lose focus.
Many kids simply hit their talent potential earlier because they got a head start on hard training.
Any kid who can run under 8:50 should have a reasonable shot of being a 13:20/27:40 type in the future but of those that had not all went on to do that. I refuse to believe that the mileage they ran at a young age had any thing to do with that personally. You have to stay in this sport a long time, handle adversity well, stay optimistic and relatively injury free and not everyone is mentally and emotionally strong enough for this to occur. There are many life adjustments and stresses to experience between the ages of 16 and mid to late 20's and how kids cope with these will greatly determine how their running careers go. Not all are able to handle these experiences as well as others and their running suffers as a byproduct.
You cannot predict what any 18-year old's future will hold, whether they go on to greatness or obscurity.