My 12-year-old son is starting to “dabble” in competitive junior tennis. I say dabble because I am quickly learning that much of the opposition plays tennis full time and year-round. The vast majority of these players are not involved in any other sport. Tennis is it. My son, on the other hand, currently plays several sports. Personally, I think it is a good thing to be diverse at 12. Yet, by playing multiple sports it can become a challenge to be successful, especially in today’s competitive …