Great topic. I think it is hard to peg a specific age and say it works for all children. And you would have to define "specialization". Do you mean focusing on one sport to the exclusion of all others or just giving a little extra attention to one sport versus another. I tend to think that a little extra sports instruction can be a good thing for athletes of any age. However, if you are going to cut off all other sports and focus on one, there could be psychology byproducts of that much intensity. I think you would want to see some visible signals from your child that he is ready for that type of focus.