Since 2008, participation in triathlon competitions has risen 60% in the United States. Interestingly, more than 850,000 adults over 40 years of age currently participate in them. The fastest growing age groups among the competitors are those aged 40 to 44 and those 35 to 39.
At the Ironman triathlon distance event, the number of finishers over 40 years of age has increased from 25% to more than 50% over the last quarter century.
-Source: Journal of the American Academy of Orthopaedic Surgeons, September 2014