Many years ago a coach of mine explained to me that it takes approximately seven years to get to a proficient level at any sport. Take gymnastics for example. When you see our Olympic team, their ages vary from 12 to 16 years old. Many of these athletes started when they were 3 to 4 years old. Many of these gymnasts peak in their teens and really never return to the sport in a competitive nature.
It was not uncommon 10 to 15 years ago for many of my students to "burn-out" (over play, get bored, etc.). In recent years, my idea of what is important has changed.
Now, I prepare players for a lifetime of having fun and enjoying tennis. When you train players to peak at a certain time in their career, I believe that you add undo pressure and take away most of the enjoyment. Only a very tiny percentage of players you teach will even go to college playing tennis, much less make it on the tour.
Start taking lessons early. Fundamentals are necessary for cutting down on injuries and increase longevity in the game. Also, encourage your students to have interests in several areas. Variety is the spice of life. Cross training will add more fun and interest. During times of injury, players that cross-train have more avenues to recovery. View the sport as providing a lifetime of enjoyment. I try to teach my students that they can maintain a high level of competitive tennis without going out everyday and mindlessly hitting millions of balls. When you are on the court, focus on tennis. When you are not on the court, forget about tennis. Focus on what you are doing at the time. This way, I believe, players will always maintain a deep love of the sport of tennis for a lifetime.
Good luck on the court!