Triathlon is all about determination and focus. Take 70 year old Ken Murley for example. Not only did he spend last summer doing every race he could to qualify for three separate Australian teams for World Championships at sprint distance (750m swim/ 20km bike/ 5km run), standard distance (twice that) and half-Ironman (1900m/90km/21km), but he spent the past two weeks WINNING the World Championships in each of those three races.
A World Champion three times over in 9 days. Nothing is impossible, no matter what age you are if you put your mind to it.
