When following the Norseman Xtreme triathlon online last month, I googled Graeme Stewart, who lead the race for quite some time, and came across his blog. A post of his pointed me to an interesting article about the age and performance of the top ten finishers in the yearly Ironman World Championships in Kona, Hawaii. The authors (Gallmann, Knechtle, Rüst, Rosemann, Lepers ) first show that as in many other sports, the finishing times (of the top 10) have improved considerably the last 30 years (1982-2012): 

This can be due to many factors. The surprising finding is that concurrently with going faster, the top finishers have become older. The average age of the top 10 men increased by 7 years, from 27 in 1983 to 34 in 2012!

The authors speculate that experience may have become more important with time or that the race may have become more attractive to former professional athletes, but do not know the answer. The point Stewart makes is that the results suggest that it takes time to build endurance and that for most people there is room for improvement regardless of age.