Consistent moderate workouts over time produce the best results. Workouts should be scientifically sound. Young athletes will naturally get stronger and faster as their bodies mature. Athletes that follow training plans and don't skip workouts will be instilled with a good work ethic and see that their positive results come from dedication and discipline. Athletes who are wishy washy might get a different message that I am not willing to send.
For the competitive, focus on speed development while young, but if necessary, let them experience success in the distance races.
For the non-competitive and the competitive, develop healthy habits and understanding for a healthy and active lifestyle.
Encourage cross-training in young athletes to develop muscle balance, over all athleticism, and active lifestyle.