Just read an article about Early Specialization. I also read the comments for the article, and this is quite the topic. As more sports are moving to being a year round sport, I can see how more people would defend early specialization, even though the anecdotal evidence says that it may not be the best thing for young athletes.
As a professional swim coach of young athletes, it would be in my best interest to defend early specialization, but really I am torn on the subject. I do believe that 12 and unders should participate in multiple activities, but at the same time, for those kids who really do enjoy a single sport, or if the family can only afford or have time to do a single activity, then I am ok with that as well.
Parents ask me when a child should start deciding what they want to participate in. My answer is this, for 10 and under athletes a wide range of activities is fine. They may not excel at any one sport or activity due to this, but if you keep that in perspective it'll be ok. At 11 a child should narrow down the sports or activities that they enjoy the most, and begin to concentrate more on those few activities. Here is where you should see some progress towards the higher levels of the sport for their respective age. Between 12 and 13 a child should come to decision on the few things that they wish pursue. This could be two activities, and you don't have to focus on a single activity.
I give this as a generalization in terms of swimming. Other sports are going to tell you different things because there are still those sports that believe that if you aren't at a high level by 9, then you won't make it in their sport. Swimming is not this way. Staying active throughout the year and getting skill development and maintaining adaptation to the water helps build skills. At the ages around 11 or 12 the swimmer will begin to take those skills and strengthen them. They will also begin to get a little more aerobic training which will increase the repetitions of the skills previously developed. At 13 or 14 is when the aerobic training is needed and a bigger time commitment is necessary. Looking at the development you can see how I came up with the narrowing of activities by age according to the time needed to progress in swimming. This is assuming that they have been doing skill development since they were 7 or 8 years old. Joining swimming later will delay the development, as they will need to develop the skills before high repetitions, or you could develop bad habits and continue to increase the myelin sheath for that particular incorrect movement.
Now back to my thoughts on early specialization in general. I think that programs need to design themselves with early specialization child in mind. This means recognizing the appropriate skill development and physical development good for various ages. Not enough training is incorporated with child development, rather sports just see training as training, regardless of age. This is not the case, and this is where sports get in trouble and create the mental or emotional "burn out."
Developing young athletes is to prepare them for when high performance matters. We over emphasize youth sports and create national competitions of youth sports spectacles and high moments of a child's life. I like the idea of localized development with the occasional travel to get some new competition. It is as the Senior or High School level where traveling to more national like events is best suited. High School age is when the swimmer will either be ending their swimming career or showcasing themselves to be able to compete at the next level. So in the development process of a young athlete you must spend years developing skills; years strengthening and refining those skills; years developing the body physiologically; years of competitions to prepare them for what it is like in a race; and mentally preparing the athletes for the roller coaster ride that a swim career is like and the work ethic and mind set to deal with those ups and downs.