This depends entirely on the duration/intensity of the training (and to a lesser degree, the age of the athlete).
Consult any sport nutritional research. Glycogen restoration is a primary concern IMMEDIATELY following exercise (again, depending on intensity) and protein is also crucial for repairation. High school aged athletes (is that what you meant by 'young athletes') should be consuming high sugary carbs post workout combined with a quality protein source (preferably in liquid form). The sugar is necessary to illicit an insulin response, which will serve to push both the sugar and the protein into storage (thereby fufilling the restoration and repairation requirements).
If the activity is long, or performed in excessive heat, than electrolyte concerns are very real and need to be addressed. Pedia-lyte is a great supplement for these situations (formulated for infants/toddlers).
Specific references can be found through sport nutrition research as to certain percentages of carbs/proteins based on age, activity level, nutritional needs and duration.
There is no real answer to the question. It depends far too much on certain variables I mentioned. Water can (and should) be the answer a lot of the times, but again... it depends.
Hope this helped!
- Brian Grasso