A majority, in fact 77.7%, of high school athletic directors report an increasing trend in sports specialization. Further evidence for early sports specialization is the growing number of travel leagues at 7 or 8 years of age.
A study of US Tennis Association junior tennis players found that 70% began specializing at an average age of 10.4 years old. Specialization rate gradually increased after 14 years, with 95% of players by age 18 years. However, enjoyment and satisfaction ratings decreased in players older than 14 years old.
Source: Sports Health, May-June 2013