Competitive-grouping by chronological age is a common organizational strategy in competition which may unintentionally promote relative age effects, for the benefit of older individuals within the same competitive-group, especially in young athletes. This work presents the aim of analyzing differences in young triathletes on their performance within each competitive group. A total of 1243 entries of both sexes, both children—13–14 years old—and cadets—15–17 years old—participated. Firstly, we identified the year in the competitive group and relative-age semester for all the triathletes who competed in a total of six seasons from 2013 to 2018. Secondly, the performance indicator was calculated in all the segments in a triathlon competition f...